Method for producing porous polyimide film, and porous polyimide film

ABSTRACT

There is provided a method for producing a porous polyimide film, including a first step of forming a coating film containing a polyimide precursor solution where a polyimide precursor and an organic amine compound are dissolved in an aqueous solvent, and a resin particle incapable of dissolving in the polyimide precursor solution, followed by drying of the coating film to form a coat containing the polyimide precursor and the resin particle, and a second step of heating the coat to imidize the polyimide precursor and form a polyimide film, the second step including a treatment for removing the resin particle.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional application of U.S. patent applicationSer. No. 14/811,654, filed on Jul. 28, 2015, which is based on andclaims priority under 35 USC 119 from Japanese Patent Application No.2015-064679, Japanese Patent Application No. 2015-64680, and JapanesePatent Application No. 2015-064681, all filed on Mar. 26, 2015.

BACKGROUND

1. Field

The present invention relates to a method for producing a porouspolyimide film, and a porous polyimide film.

2. Description of the Related Art

A polyimide resin is a material having excellent properties in terms ofmechanical strength, chemical stability and heat resistance, and aporous polyimide film having such properties is attracting attention.

SUMMARY

[1] A method for producing a porous polyimide film, containing:

a first step of forming a coating film containing a polyimide precursorsolution where a polyimide precursor and an organic amine compound aredissolved in an aqueous solvent, and a resin particle incapable ofdissolving in the polyimide precursor solution, followed by drying ofthe coating film to form a coat containing the polyimide precursor andthe resin particle, and

a second step of heating the coat to imidize the polyimide precursor andform a polyimide film, the second step including a treatment forremoving the resin particle.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A, FIG. 1B, FIG. 1C, and FIG. 1D are process charts showing anexample of the production method of a porous polyimide film in anexemplary embodiment of the present invention.

FIG. 2A, FIG. 2B, FIG. 2C, and FIG. 2D are process charts showinganother example of the production method of a porous polyimide film inan exemplary embodiment of the present invention.

FIG. 3A, FIG. 3B, and FIG. 3C are process charts showing still anotherexample of the production method of a porous polyimide film in anexemplary embodiment of the present invention.

FIG. 4A, FIG. 4B, FIG. 4C, and FIG. 4D are process charts showing anexample of the production method of a porous polyimide film in anexemplary embodiment of the present invention.

FIG. 5A, FIG. 5B, FIG. 5C, and FIG. 5D are process charts showinganother example of the production method of a porous polyimide film inan exemplary embodiment of the present invention.

FIG. 6A, FIG. 6B, and FIG. 6C are process charts showing still anotherexample of the production method of a porous polyimide film in anexemplary embodiment of the present invention.

FIG. 7A, FIG. 7B, and FIG. 7C are process charts showing an example ofthe production method of a porous polyimide film in an exemplaryembodiment of the present invention.

DESCRIPTION OF REFERENCE NUMERALS AND SIGNS

-   1: Resin particle-   2: Binder resin-   3: Base plate-   4: Release layer-   5: Polyimide precursor solution-   7: Vacancy-   11: Uncrosslinked resin particle-   61: Polyimide coat-   62: Porous polyimide film

DETAILED DESCRIPTION

Exemplary embodiments as an example of the present invention aredescribed below.

At first, a first exemplary embodiment of the present invention isdescribed below.

<Production Method of Porous Polyimide Film>

The production method of a porous polyimide film according to the firstexemplary embodiment of the present invention includes a first step offorming a coating film containing a polyimide precursor solution where apolyimide precursor and an organic amine compound are dissolved in anaqueous solvent, and a resin particle incapable of dissolving in thepolyimide precursor solution, followed by drying of the coating film toform a coat containing the polyimide precursor and the particle, and asecond step of heating the coat to imidize the polyimide precursor andform a polyimide film, the second step including a treatment forremoving the resin particle.

The “incapable of dissolving in” as used in the first exemplaryembodiment of the present invention means that the object substancesubstantially maintains the form of a resin particle in the objectliquid at 25° C., and encompasses a case where the object substancedissolves in an amount of 3 mass % or less.

In the production method of a porous polyimide film according to thefirst exemplary embodiment of the present invention, thanks to theconfiguration above, the production process is simplified, compared witha case where the solvent in the polyimide precursor solution is only anorganic solvent.

The polyimide film is obtained, for example, by applying a polyimideprecursor solution dissolved in an organic solvent (e.g.,N-methylpyrrolidone (hereinafter, sometimes referred to as “NMP”)) or apolyimide precursor solution dissolved in a high-polarity solvent suchas N,N-dimethylacetamide (hereinafter, sometimes referred to as “DMAc”),and then heating and shaping the coating.

The porous polyimide film is obtained, for example, by a method where afilm is shaped using a polyimide precursor solution dissolved in anorganic solvent, then contacted with a poor solvent such as water toprecipitate a polyamic acid, thereby creating pores, and thereafter,subjected to imidization, or a method of using a particle and apolyimide precursor solution dissolved in an organic solvent andremoving the particle. The production method of a porous polyimide filmincludes, for example, a method of obtaining a porous polyimide filmhaving formed therein a three-dimensionally ordered array structure(3DOM structure) of vacancies by using a silica particle layer as amold, and a method of producing a coat by using a polyimide precursorsolution having dispersed therein silica particles, firing the coat, andthen removing the silica particles to obtain a porous polyimide film.However, according to these methods, a chemical such as hydrofluoricacid needs to be used in the treatment for removing the silica particle.In addition, in the case of producing a silica particle layer mold, ahigh heat treatment at 1,000° C. or more must be performed for sinteringthe particles together to form the silica particle layer. In the case ofusing a polyimide precursor solution having dispersed therein silicaparticles, a treatment for enhancing the dispersibility of the silicaparticle in the polyimide precursor solution is sometimes performed bysubjecting the surface of the silica particle to a hydrophobizationtreatment. Therefore, in these production methods, the productivity islow, and the cost is high.

The production method also includes a method of forming a film from aresin composition containing a resin particle and a polyimide precursorsolution dissolved in an organic solvent, and then removing the resinparticle to obtain a porous polyimide film. In this case, since NMP orDMAc has a very high dissolving power, a general resin particle (forexample, a polystyrene resin particle) is subject to dissolution, etc.due to the organic solvent dissolving the polyimide precursor, and aporous polyimide film may be hardly obtained.

Meanwhile, there is a method of producing, as a resin particle, adispersion liquid of a polyoxyalkylene resin that is difficult todissolve in an organic solvent such as NMP, mixing the dispersion liquidwith a polyimide precursor solution dissolved in an organic solvent toprepare a resin composition, forming a film from the resin composition,and then removing the resin particle by heating to obtain a porouspolyimide film. Since this resin particle is produced by emulsionpolymerization, etc., a step of replacing the solvent by an organicsolvent such as NMP may have to be provided so as to mix the resinparticle with the polyimide precursor solution.

On the other hand, in the production method of a porous polyimide filmin the first exemplary embodiment of the present invention, a polyimideprecursor is dissolved in an aqueous solvent. Because of an aqueoussolvent, the resin particle is kept from dissolution, etc. andtherefore, a porous polyamide film is easily produced. In addition, astep of replacing the dispersion solvent in the water dispersion liquidof the resin particle by an organic solvent need not be provided.

The production method of a porous polyimide film in the first exemplaryembodiment of the present invention encompasses a method where a resinparticle layer working out to a mold is produced using a resin particle,but at the time of production of the resin particle layer, a heatingtemperature (for example, 100° C.) capable of forming a resin particlelayer while maintaining the shape of the resin particle may besufficient. Therefore, a resin particle layer mold is produced withoutperforming a high heat treatment at 1,000° C. or more required whenproducing a silica particle layer mold, and the production process issimplified.

For these reasons, it is considered that in the production method of aporous polyimide film according to the first exemplary embodiment of thepresent invention, the production process is simplified, compared with acase where the solvent in the polyimide precursor solution is only anorganic solvent.

In the porous polyimide film obtained by the production method of aporous polyimide film in the first exemplary embodiment of the presentinvention, generation of cracks is likely suppressed. This is presumedto occur because in the production method of a porous polyimide film inthe first exemplary embodiment of the present invention, a resinparticle is used and the use thereof effectively contributes to therelaxation of residual stress in the imidization step of the polyimideprecursor.

In the porous polyimide film obtained by the production method of aporous polyimide film in the first exemplary embodiment of the presentinvention, the variation in vacancy shape, vacancy diameter, etc. islikely suppressed. The reason therefor is presumed to be that a resinparticle is used and the use thereof effectively contributes to therelaxation of residual stress in the imidization step of the polyimideprecursor.

Furthermore, in the porous polyimide film obtained by the productionmethod of a porous polyimide film in the first exemplary embodiment ofthe present invention, the polyimide precursor is dissolved in anaqueous solvent and therefore, the boiling temperature of the polyimideprecursor solution is about 100° C. Accordingly, the solvent rapidlyvolatilizes as the coat containing the polyimide precursor and the resinparticle is heated, and thereafter, an imidization reaction proceeds.Before deformation of the resin particle in the coat occurs due to heat,the resin particle loses fluidity and becomes insoluble in an organicsolvent. For this reason, the shape of vacancy is considered to belikely maintained, making it easy to suppress the variation in vacancyshape, vacancy diameter, etc. In addition, thanks to an aqueous solvent,the resin particle is prevented from dissolution, etc. Accordingly, thevacancy diameter of the porous polyimide film can be controlled by theparticle diameter of the resin particle.

Incidentally, in the case of using a silica particle, it is thought thatsince volume contraction is hardly absorbed in the imidization step, thepolyimide film after imidization is prone to generation of cracks. Inaddition, in the case of using a silica particle, it is thought thatsince a chemical such as hydrofluoric acid is used, an ion is likely toremain as an impurity.

The production method of a porous polyimide film according to the firstexemplary embodiment of the present invention is described below.

In the drawings referred to in the description of the production method,the same numerical reference is used for the same component part. As tothe numerical reference in each drawing, 1 indicates a resin particle, 2indicates a binder resin, 3 indicates a base plate, 4 indicates arelease layer, 5 indicates a polyimide precursor solution, 7 indicates avacancy, 61 indicates a coat (polyimide coat) in the process ofperforming imidization of the polyimide precursor, and 62 indicates aporous polyimide film.

The production method of a porous polyimide film according to the firstexemplary embodiment of the present invention includes theabove-described first step and the above-described second step. In thefollowing, the production method depicted in FIG. 1A, FIG. 1B,

FIG. 1C, and FIG. 1D (one example of the production method according tothe first exemplary embodiment of the present invention) is described,but the production method is not limited thereto.

(First Step)

In the first step, a polyimide precursor solution where a polyimideprecursor and an organic amine compound are dissolved in an aqueoussolvent, is prepared. Thereafter, a coating film containing thepolyimide precursor solution and a resin particle incapable ofdissolving in the polyimide precursor solution is formed on a baseplate. The coating film formed on the base plate is then dried to form acoat containing the polyimide precursor and the resin particle.

In the first step, the method for forming, on a base plate, a coatingfilm containing the polyimide precursor solution and a resin particleincapable of dissolving in the polyimide precursor solution includesspecifically the following method.

First, a resin particle dispersion liquid containing a resin particleincapable of dissolving in the polyimide precursor solution, an organicsolvent incapable of dissolving the resin particle, and a binder resincapable of dissolving in the organic solvent is prepared. Next, theresin particle dispersion liquid is applied onto a base plate and driedto form a resin particle layer. In the resin particle layer formed onthe base plate, for example, adjacent resin particles are presentwithout dissolving each other, adjacent resin particles are at the sametime bonded to each other with a binder resin, and a void is formedbetween resin particles of the resin particle layer (see, FIG. 1A).

Meanwhile, a polyimide precursor solution where a polyimide precursorand an organic amine compound are dissolved in an aqueous solvent, ispreviously prepared.

The previously prepared polyimide precursor solution is impregnatedbetween resin particles of the resin particle layer formed on the baseplate. By impregnating the polyimide precursor solution between resinparticles of the resin particle layer, the void formed between resinparticles of the resin particle layer is filled with the polyimideprecursor solution. In order to promote filling, it is also preferableto remove a gas component in the void by reducing the pressure in thestate of the polyimide precursor solution being put into contact withthe resin particle. Thereafter, the coating film is dried, whereby acoat containing the polyimide precursor and the resin particle is formedon the base plate (see, FIG. 1B).

The base plate on which the coat containing the polyimide precursor andthe resin particle is formed, is not particularly limited and includes,for example, a base plate made of a resin such as polystyrene andpolyethylene terephthalate; a glass-made base plate; a ceramic-made baseplate; a metal base plate such as iron and stainless steel (SUS); and acomposite material base plate formed by combining these materials. Ifdesired, the base plate may be subjected to a release treatment with asilicone-based or fluorine-based release agent, etc. to provide arelease layer.

The method for producing the resin particle dispersion liquid is notparticularly limited and includes, for example, a method where each ofthe resin particle incapable of dissolving in the polyimide precursorsolution, the organic solvent incapable of dissolving the resinparticle, and the binder resin capable of dissolving in the organicsolvent is weighed and these are mixed and stirred to obtain the resinparticle dispersion liquid. As for the resin particle, a dispersionliquid where resin particles are previously dispersed may be produced,or a commercial product where resin particles are previously dispersedmay be prepared. In the case of producing a dispersion liquid whereresin particles are previously dispersed, the dispersibility of theresin particle may be increased, for example, by using at least eitherone of an ionic surfactant and a nonionic surfactant.

The binder resin may be previously dissolved in the above-describedorganic solvent or may be mixed with the resin particle and the organicsolvent and dissolved.

The ratio (mass ratio) between the resin particle and the binder resinin the resin particle dispersion liquid is suitably resin particle:binder resin=from 100:0.5 to 100:50, preferably from 100:1 to 100:30,more preferably from 100:2 to 100:20. Within this range, in the resinparticle layer formed from the resin particle dispersion liquid, a statewhere the surface of each resin particle is partially or entirelycovered with the binder resin and adjacent resin particles are bonded toeach other (including a primarily adhering state; a so-calledpseudo-adhesion state), is likely formed, and a void producing an airlayer state is readily formed between resin particles of the resinparticle layer.

The resin particle is not particularly limited as long as it does notdissolve in the polyimide precursor solution, and includes, for example,a resin particle obtained by polycondensation of a polymerizablemonomer, such as polyester resin and urethane resin, and a resinparticle obtained by radical polymerization of a polymerizable monomer,such as vinyl resin and olefin resin. The resin particle obtained byradical polymerization includes, for example, resin particles of(meth)acrylic resin, (meth)acrylic acid ester resin,styrene.(meth)acrylic resin, polystyrene resin and polyethylene resin.

Among these, the resin particle is preferably at least one memberselected from the group consisting of (meth)acrylic resin, (meth)acrylicacid ester resin, styrene.(meth)acrylic resin, and polystyrene resin.

The resin particle may or may not be crosslinked. From the standpoint ofeffectively contributing to the relaxation of residual stress in theimidization step of the polyimide precursor, an uncrosslinked resinparticle is preferred.

The term “(meth)acrylic” as used in an exemplary embodiment of thepresent invention means that both “acrylic” and “methacrylic” areencompassed.

In the case where the resin particle is, for example, a vinyl resinparticle, the synthesis method thereof is not particularly limited, anda known polymerization method (a radical polymerization method such asemulsion polymerization, soap-free emulsion polymerization, suspensionpolymerization, miniemulsion polymerization and microemulsionpolymerization) may be applied.

For example, in the case of applying an emulsion polymerization methodto the production of the vinyl resin particle, a monomer such asstyrenes and (meth)acrylic acids is added to water having dissolvedtherein a water-soluble polymerization initiator such as potassiumpersulfate and ammonium persulfate, a surfactant such as sodiumdodecylsulfate and diphenyl oxide disulfonates is further added, ifdesired, and the mixture is heated under stirring to performpolymerization, whereby the vinyl resin particle is obtained.

As for the monomer, the vinyl resin includes, for example, a vinyl resinunit obtained by polymerization of a monomer, e.g., styrenestructure-containing styrenes such as styrene, an alkyl-substitutedstyrene (for example, α-methylstyrene, 2-methylstyrene, 3-methylstyrene,4-methylstyrene, 2-ethylstyrene, 3-ethylstyrene and 4-ethylstyrene), ahalogen-substituted styrene (for example, 2-chlorostyrene,3-chlorostyrene and 4-chlorostyrene) and divinylnaphthalene; vinylgroup-containing esters such as methyl (meth)acrylate, ethyl(meth)acrylate, n-propyl (meth)acrylate, n-butyl (meth)acrylate, lauryl(meth)acrylate, 2-ethylhexyl (meth)acrylate and trimethylolpropanetrimethacrylate (TMPTMA); vinyl nitriles such as acrylonitrile andmethacrylonitrile; vinyl ethers such as vinyl methyl ether and vinylisobutyl ether, vinyl ketones such as vinyl methyl ketone, vinyl ethylketone and vinyl isopropenyl ketone; acids such as (meth)acrylic acid,maleic acid, cinnamic acid, fumaric acid and vinylsulfonic acid; andbases such as ethyleneimine, vinylpyridine and vinylamine.

As another monomer, a monofunctional monomer such as vinyl acetate, abifunctional monomer such as ethylene glycol dimethacrylate, nonanediacrylate and decanediol diacrylate, or a polyfunctional monomer suchas trimethylolpropane triacrylate and trimethylolpropanetrimethacrylate, may be used in combination.

The vinyl resin may be a resin using such a monomer alone or a resinthat is a copolymer using two or more of these monomers.

As described above, the resin particle is preferably uncrosslinked, butin the case of crosslinking the resin particle, when a crosslinkingagent is used at least as part of the monomer components, the ratio ofthe crosslinking agent to all monomer components is preferably from 0mass % to 20 mass %, more preferably from 0 mass % to 5 mass %, stillmore preferably 0 mass %.

In the case where the monomer used for the resin constituting the vinylresin particle contains styrene, the ratio of styrene to all monomercomponents is preferably from 20 mass % to 100 mass %, more preferablyfrom 40 mass % to 100 mass %.

The average particle diameter of the resin particle is not particularlylimited but is suitably, for example, 2.5 μm or less, preferably 2.0 μmor less, more preferably 1.0 μm or less. The lower limit is notparticularly limited but is suitably 0.001 μm or more, preferably 0.005μm or more, more preferably 0.01 μm or more.

As for the average particle diameter of the resin particle, a cumulativedistribution for the volume is drawn from the small diameter side withrespect to divided particle size ranges (channels) by using a particlesize distribution obtained by measurement by means of a laserdiffraction particle size distribution measuring apparatus (for example,LA-700, manufactured by Horiba, Ltd.), and the particle diameter at anaccumulation of 50% relative to all particles is measured as the volumeaverage particle diameter D50v.

The resin particle may be a commercial product. Specifically, thecrosslinked resin particle includes, for example, a crosslinkedpolymethyl methacrylate (MBX-Series, produced by Sekisui Plastics Co.,Ltd.), a crosslinked polystyrene (SBX-Series, produced by SekisuiPlastics Co., Ltd.), and a crosslinked methyl methacrylate-styrenecopolymer resin particle (MSX-Series, produced by Sekisui Plastics Co.,Ltd.).

The uncrosslinked resin particle includes, for example, a polymethylmethacrylate (MB-Series, produced by Sekisui Plastics Co., Ltd.), and a(meth)acrylic acid ester-styrene copolymer (FS-Series, produced byNippon Paint Co., Ltd.).

The binder resin is not particularly limited as long as it dissolves inthe organic solvent and does not dissolve in the polyimide precursorsolution. The binder resin includes, for example, an acetal resin suchas polyvinylbutyral resin; a polyamide resin such as nylon; a polyesterresin such as polyethylene terephthalate and polyethylene naphthalate; apolyolefin resin such as polyethylene and polypropylene; an acrylicresin; a vinyl resin such as polyvinyl chloride resin and polyvinylidenechloride resin; a polyurethane resin; polyvinylpyrrolidone, polyethyleneglycol, and polyvinyl alcohol. A polyvinylacetal resin and an aliphaticpolyamide resin are preferred.

The organic solvent incapable of dissolving the resin particle includes,for example, alcohols such as methanol, ethanol and ethylene glycol;cellosolves such as ethylene glycol monomethyl ether, hydrocarbons suchas hexane; ketones such as acetone; aromatics such as toluene; esterssuch as ethyl acetate; and nitriles such as acetonitrile.

Among these, from the standpoint of maintaining the shape of the resinparticle, alcohols and cellosolves are preferred, and the binder resinis preferably a resin soluble in alcohols and cellosolves (for example,acetal resin and polyamide resin).

The method for applying the resin particle dispersion liquid onto a baseplate is not particularly limited and includes various methods, forexample, a spray coating method, a spin coating method, a roll coatingmethod, a bar coating method, a slit die coating method, and an inkjetcoating method.

The coating film formed by applying the resin particle dispersion liquidonto the base plate is dried to obtain the resin particle layer. Thedrying temperature may be a temperature capable of maintaining the shapeof the resin particle and bonding resin particles to each other (forexample, 100° C.).

Subsequently, a coating film containing the polyimide precursor solutionand the resin particle is formed by impregnating the previously preparedpolyimide precursor solution between resin particles of the resinparticle layer formed above, and the coating film is then dried to forma coat containing the polyimide precursor and the resin particle.

The method for impregnating the polyimide precursor solution is notparticularly limited and includes, for example, a method where the baseplate having formed thereon the resin particle layer is dipped in thepolyimide precursor solution, and a method where the polyimide precursorsolution is applied onto the resin particle layer formed on the baseplate and impregnated between particles of the resin particle layer.

The method for applying the polyimide precursor solution onto the resinparticle layer formed on the base plate includes various methods, forexample, a spray coating method, a spin coating method, a roll coatingmethod, a bar coating method, a slit die coating method, and an inkjetcoating method. From the standpoint of impregnating the polyimideprecursor solution between resin particles forming the resin particlelayer, a vacuum impregnation filling method of applying the polyimideprecursor solution onto the resin particle layer and then reducing thepressure, thereby filling between resin particles with the polyimideprecursor solution, is preferably employed, because the polyimideprecursor solution is efficiently impregnated into a void between resinparticles.

The method for forming a coating film containing the polyimide precursorsolution and the resin particle is not limited to the methods above.

For example, the method specifically includes the following method.First, a polyimide precursor solution where a polyimide precursor and anorganic amine compound are dissolved in an aqueous solvent, is prepared.Next, the polyimide precursor solution and a resin particle incapable ofdissolving in the polyimide precursor solution are mixed to obtain apolyimide precursor solution having dispersed therein resin particles(hereinafter, sometimes referred to as “resin particle-dispersedpolyimide precursor solution”). This resin particle-dispersed polyimideprecursor solution is applied onto the base plate to form a coating filmcontaining the polyimide precursor solution and the resin particle.Resin particles in the coating film are distributed in the state ofaggregation being suppressed (see, FIG. 3A). Thereafter, the coatingfilm is dried, whereby a coat containing the polyimide precursor and theresin particle is formed on the base plate.

The method for producing the resin particle-dispersed polyimideprecursor solution is not particularly limited and includes, forexample, a method of mixing the polyimide precursor solution and theresin particle in a dry state, and a method of mixing the polyimideprecursor solution with a dispersion liquid where resin particles arepreviously dispersed in an aqueous solvent. As the dispersion liquidwhere resin particles are previously dispersed in an aqueous solvent, aresin particle dispersion liquid where resin particles are previouslydispersed in an aqueous solvent may be produced, or a commerciallyavailable dispersion liquid where resin particles are previouslydispersed in an aqueous solvent may be prepared. In the case ofproducing a dispersion liquid where resin particles are previouslydispersed, the dispersibility of the resin particle may be increased,for example, by using at least either one of an ionic surfactant and anonionic surfactant.

In the polyimide precursor solution having dispersed therein resinparticles, the ratio of the resin particle is suitably, in terms of massratio assuming that the solid content of the polyimide precursorsolution is 100, solid content of polyimide precursor solution: resinparticle=from 100:20 to 100:200, preferably from 100:25 to 100:180, morepreferably from 100:30 to 100:150.

The method for applying the resin particle-dispersed polyimide precursorsolution onto the base plate is not particularly limited and includesvarious methods, for example, a spray coating method, a spin coatingmethod, a roll coating method, a bar coating method, a slit die coatingmethod, and an inkjet coating method.

The amount of the polyimide precursor solution coated for obtaining thecoating film containing the polyimide precursor solution obtained aboveand the resin particle is suitably an amount allowing the resin particleto be exposed on the coating film surface, because the pore area ratioof the porous polyimide film can be increased. For example, in the caseof impregnating the polyimide precursor solution between resin particlesforming the resin particle layer, the polyimide precursor solution issuitably impregnated with a thickness less than the thickness of theresin particle layer.

In the case of forming the resin particle-dispersed polyimide precursorsolution on the base plate, the solution is suitably formed after addingthe resin particle in an amount allowing the resin particle to beexposed on the coating film surface.

After the coating film containing the polyimide precursor solutionobtained by the method above and the resin particle is formed, thecoating film is dried to form a coat containing the polyimide precursorand the resin particle. Specifically, the coating film containing thepolyimide precursor solution and the resin particle is dried, forexample, by heat drying, natural drying, vacuum drying or other methodsto form a coat. More specifically, the coat is formed by drying thecoating film such that the solvent remaining in the coat accounts for50% or less, preferably 30% or less, relative to the solid content ofthe coat. This coat is in a state of the polyimide precursor beingdissolvable in water.

At the time of formation of the coating film, the coating film may beformed with an amount enough to embed the resin particle in the coatingfilm. In this case, in the first step, a treatment for exposing theresin particle may be performed in the process of drying the obtainedcoating film to form the coat, so as to provide a state of the resinparticle being exposed. The pore area ratio of the porous polyimide filmis increased by performing the treatment for exposing the resinparticle.

The treatment for exposing the resin particle specifically includes thefollowing method.

When the coating film is formed to embed the resin particle layer byimpregnating the polyimide precursor solution between resin particlesforming the resin particle layer, the polyimide precursor solution ispresent in the region exceeding the thickness of the resin particlelayer (see, FIG. 1B).

In the process of drying the coating film to form the coat containingthe polyimide precursor and the resin particle after obtaining thecoating film containing the polyimide precursor solution and the resinparticle, the coat is in a state of the polyimide precursor beingdissolvable in water. The coat in this state is treated, for example, bywiping or dipping in water, whereby the resin particle can be exposed.Specifically, the polyimide precursor solution present in the regionexceeding the thickness of the resin particle layer is subjected to, forexample, a treatment for exposing the resin particle layer by wetwiping, whereby the polyimide precursor solution present in the regionexceeding the thickness of the resin particle layer is removed. As aresult, the resin particle present in the region at the top of the resinparticle layer (that is, the region on the side distant from the baseplate of the resin particle layer) is exposed on the coat surface (see,FIG. 1C).

In this connection, in the case of forming the coat on the base plate byusing the resin particle-dispersed polyimide precursor solution, when acoat having embedded therein the resin particle is formed, the sametreatment as the above-described treatment for exposing the resinparticle can also be employed as the treatment for exposing the resinparticle embedded in the coat.

(Second Step)

The second step is a step of heating the coat containing the polyimideprecursor and the resin particle, obtained in the first step, to imidizethe polyimide precursor and form a polyimide film. The second stepincludes a treatment for removing the resin particle. A porous polyimidefilm is obtained through the treatment for removing the resin particle.

In the second step, the step of forming a polyimide film is specificallyperformed by heating the coat containing the polyimide precursor and theresin particle, obtained in the first step, thereby allowing imidizationto proceed, and further heating the coat to form a polyimide film. Asthe imidization proceeds and the imidization ratio rises, the coatbecomes hardly dissolvable in an organic solvent.

Thereafter, in the second step, a treatment for removing the resinparticle is performed. As for the removal, the resin particle may beremoved in the process of imidizing the polyimide precursor by heatingthe coat or may be removed from a polyimide film after the completion ofimidization (after imidization).

In the first exemplary embodiment of the present invention, the processof imidizing the polyimide precursor indicates a process of heating thecoat containing the polyimide precursor and the resin particle, obtainedin the first step, thereby allowing imidization to proceed and producinga state prior to becoming a polyimide film after the completion ofimidization.

Specifically, the resin particle is removed from the coat in the processof heating the coating film obtained in the first step, on which theresin particle is exposed, and thereby imidizing the polyimide precursor(hereinafter, the coat in this state is sometimes referred to as“polyimide coat”). Alternatively, the resin particle may be removed fromthe polyimide film after the completion of imidization. As a result, aporous polyimide film, from which the resin particle is removed, isobtained (see, FIG. 1D).

In view of removability, etc. of the resin particle, the treatment forremoving the resin particle is preferably performed when the imidizationratio of the polyimide precursor in the polyimide coat is 30% or more,in the process of imidizing the polyimide precursor. When theimidization ratio becomes 30% or more, the coat becomes hardlydissolvable in an organic solvent.

The treatment for removing the resin particle includes, for example, amethod of removing the resin particle by heating, a method of removingthe resin particle with an organic solvent capable of dissolving theresin particle, and a method of removing the resin particle bydecomposition using a laser, etc. Among these, a method of removing theresin particle by heating, and a method of removing the resin particlewith an organic solvent capable of dissolving the resin particle arepreferred.

As the method of removing the resin particle by heating, the resinparticle may be decomposed and removed, for example, by the heatingperformed for allowing the imidization to proceed in the process ofimidizing the polyimide precursor. In this case, an operation ofremoving the resin particle with a solvent is omitted, which isadvantageous in view of reducing the number of steps. On the other hand,depending on the kind of the resin particle, a decomposition gas may begenerated by heating, and rupture, cracking, etc. may occur in theporous polyimide film due to the decomposition gas. Therefore, in thiscase, a method of removing the resin particle with an organic solventcapable of dissolving the resin particle is preferably employed.Incidentally, it is also effective to further perform heating after theremoval with an organic solvent capable of dissolving the resin particleand thereby raise the removal ratio.

The method of removing the resin particle with an organic solventcapable of dissolving the resin particle includes, for example, a methodof bringing the coat into contact with an organic solvent capable ofdissolving the resin particle (for example, dipping in the solvent), andthereby dissolving and removing the resin particle. Dipping in thisstate in the solvent is preferred because the dissolution efficiency forthe resin particle is increased.

The organic solvent capable of dissolving the resin particle forremoving the resin particle is not particularly limited as long as it isan organic solvent incapable of dissolving the polyimide coat and theimidization-completed polyimide film and capable of dissolving the resinparticle. The organic solvent includes, for example, ethers such astetrahydrofuran; aromatics such as toluene; ketones such as acetone; andesters such as ethyl acetate.

In the case where the aqueous solvent remains at the time of dissolvingthe resin particle, the aqueous solvent may dissolve in the solventcapable of dissolving the resin particle and precipitation of thepolyimide precursor may occur to produce a state similar to that in aso-called wet phase transition method, making it difficult to controlthe vacancy diameter. On this account, the resin particle is preferablydissolved and removed with an organic solvent after reducing the amountof the remaining aqueous solvent to 20 mass % or less, preferably 10mass % or less, relative to the mass of the polyimide precursor.

In the second step, the heating method when heating the coat obtained inthe first step to allow the progression of imidization and therebyobtain a polyimide film is not particularly limited and includes, forexample, a method of heating the coat in two stages. In the case oftwo-stage heating, the heating conditions specifically include thefollowing heating conditions.

As for the heating conditions in the first stage, the temperature ispreferably a temperature capable of maintaining the shape of the resinparticle. Specifically, the temperature is suitably, for example, from50° C. to 150° C., preferably from 60° C. to 140° C. The heating time issuitably from 10 minutes to 60 minutes. As the heating temperature ishigher, the heating time may be shorter.

As for the heating conditions in the second stage, heating is performed,for example, under the conditions of from 150° C. to 400° C. (preferablyfrom 200° C. to 390° C.) and from 20 minutes to 120 minutes. Under theheating conditions in these ranges, the imidization reaction furtherproceeds and a polyimide film can be formed. At the time of heatingreaction, heating is suitably performed by step-by-step raising thetemperature or gradually raising the temperature at a constant rate,before reaching the final temperature of heating.

The heating conditions are not limited to the above-described two-stageheating method and, for example, a method of heating the coat in asingle stage may be employed. In the case of the single-stage heatingmethod, for example, the imidization may be completed only under theheating conditions of the second stage above.

In the case where a treatment for exposing the resin particle is notapplied in the first step, from the standpoint of increasing the porearea ratio, a treatment for exposing the resin particle is preferablyperformed in the second step to produce a state of the resin particlebeing exposed. In the second step, the treatment for exposing the resinparticle is preferably performed in the process of performingimidization of the polyimide precursor or after imidization but beforethe treatment for removing the resin particle.

For example, in the first step, the resin particle layer is formed onthe base plate (see, FIG. 2A), and the polyimide precursor solution isimpregnated between resin particles of the resin particle layer to forma coating film in a state of the resin particle being embedded therein(see, FIG. 2B). Thereafter, a coat containing the polyimide precursorand the resin particle is formed without performing a treatment forexposing the resin particle in the process of drying the coating film toform a coat. The coat formed by this method is a coat in a state of theresin particle layer being embedded therein. Before performing thetreatment for removing the resin particle by heating, the coat issubjected to a treatment for exposing the resin particle on thepolyimide film in the process of imidizing the polyimide precursor orafter the completion of imidization (after imidization).

In the second step, the treatment for exposing the resin particleincludes, for example, a treatment applied when the polyimide coat is inthe following state.

In the case of performing the treatment for exposing the resin particlewhen the imidization ratio of the polyimide precursor in the polyimidecoat is less than 15% (i.e., a state of the polyimide coat beingdissolvable in water), the treatment for exposing the resin particleembedded in the polyimide coat includes a wiping treatment, a waterdipping treatment, etc.

In the case of performing the treatment for exposing the resin particlewhen the imidization ratio of the polyimide precursor in the polyimidecoat is 15% or more (i.e., a state of being hardly dissolvable in anorganic solvent) or when the imidization is completed to generate apolyimide film, the method includes a method of exposing the resinparticle by mechanical cutting with tools such as sandpaper, and amethod of exposing the resin particle by decomposition using a laser,etc.

For example, in the case of mechanical cutting, part of the resinparticle present in the region at the top of the resin particle layerembedded in the polyimide coat (i.e., the region on the side distantfrom the base plate of the resin particle layer) is cut together withthe polyimide coat present at the top of the resin particle, and the cutresin particle is exposed on the surface of the polyimide coat (see,FIG. 2C).

Thereafter, from the polyimide coat on which the resin particle isexposed, the resin particle is removed by the above-described treatmentfor removing the resin particle. As a result, a porous polyimide film,from which the resin particle is removed, is obtained (see, FIG. 2D).

In the case of forming a coat on the base plate by using the resinparticle-dispersed polyimide precursor solution, the resinparticle-dispersed polyimide precursor solution is applied onto the baseplate to form a coating film having embedded therein the resin particle(see, FIG. 3A). When a coat containing the polyimide precursor and theresin particle is formed without performing a treatment for exposing theresin particle in the process of drying the coating film to form a coat,a coat having embedded therein the resin particle is formed. The coat(polyimide coat) in the process of performing imidization by heating thecoat is in a state of the resin particle layer being embedded therein.As the treatment for exposing the resin particle, which is performed inthe second step so as to increase the pore area ratio, the sametreatment as the above-descried treatment for exposing the resinparticle can be employed. The resin particle is then cut together withthe polyimide coat present at the top of the resin particle to exposethe resin particle on the surface of the polyimide coat (see, FIG. 3B).

Thereafter, from the polyimide coat on which the resin particle isexposed, the resin particle is removed by the above-described treatmentfor removing the resin particle. As a result, a porous polyimide film,from which the resin particle is removed, is obtained (see, FIG. 3C).

In the second step, the base plate used in the first step for formingthe coat thereon may be separated when the coat becomes a dry coat, maybe separated when the polyimide precursor in the polyimide coat becomeshardly dissolvable in an organic solvent, or may be separated when animidization-completed film is generated.

Through these steps, a porous polyimide film is obtained. The porouspolyimide film may be post-processed according to the intended use.

[Polyimide Precursor Solution]

Respective components of the polyimide precursor solution according toan exemplary embodiment of the present invention are described below.

(Polyimide Precursor)

The polyimide precursor is a resin (polyamic acid) containing arepeating unit represented by formula (I):

(wherein A represents a tetravalent organic group, and B represents adivalent organic group).

In formula (I), the tetravalent organic group represented by A is aresidue obtained by removing four carboxyl groups from a tetracarboxylicacid dianhydride as a raw material.

The divalent organic group represented by B is a residue obtained byremoving two amino groups from a diamine compound as a raw material.

That is, the polyimide precursor containing a repeating unit representedby formula (I) is a polymer of a tetracarboxylic acid dianhydride and adiamine compound.

The tetracarboxylic acid dianhydride include both aromatic and aliphaticcompounds but is preferably an aromatic compound. In other words, thetetravalent organic group represented by A in formula (I) is preferablyan aromatic organic group.

The aromatic tetracarboxylic acid dianhydride includes, for example,pyromellitic acid dianhydride, 3,3′,4,4′-benzophenonetetracarboxylicacid dianhydride, 3,3′,4,4′-biphenylsulfonetetracarboxylic aciddianhydride, 1,4,5,8-naphthalenetetracarboxylic acid dianhydride,2,3,6,7-naphthalenetetracarboxylic acid dianhydride,3,3′,4,4′-biphenylethertetracarboxylic acid dianhydride,3,3′,4,4′-dimethyldiphenylsilanetetracarboxylic acid dianhydride,3,3′,4,4′-tetraphenylsilanetetracarboxylic acid dianhydride,1,2,3,4-furantetracarboxylic acid dianhydride,4,4′-bis(3,4-dicarboxyphenoxy)diphenyl sulfide dianhydride,4,4′-bis(3,4-dicarboxyphenoxy)diphenyl sulfone dianhydride,4,4′-bis(3,4-dicarboxyphenoxy)diphenylpropane dianhydride,3,3′,4,4′-perfluoroisopropylidene diphthalic acid dianhydride,3,3′,4,4′-biphenyltetracarboxylic acid dianhydride,2,3,3′,4′-biphenyltetracarboxylic acid dianhydride, bis(phthalicacid)phenylphosphine oxide dianhydride,p-phenylene-bis(triphenylphthalic acid) dianhydride,m-phenylene-bis(triphenylphthalic acid) dianhydride,bis(triphenylphthalic acid)-4,4′-diphenyl ether dianhydride, andbis(triphenylphthalic acid)-4,4′-diphenylmethane dianhydride.

The aliphatic tetracarboxylic acid dianhydride includes, for example, analiphatic or alicyclic tetracarboxylic acid dianhydride such asbutanetetracarboxylic acid dianhydride,1,2,3,4-cyclobutanetetracarboxylic acid dianhydride,1,3-dimethyl-1,2,3,4-cyclobutanetetracarboxylic acid dianhydride,1,2,3,4-cyclopentanetetracarboxylic acid dianhydride,2,3,5-tricarboxycyclopentylacetic acid dianhydride,3,5,6-tricarboxynorbornane-2-acetic acid dianhydride,2,3,4,5-tetrahydrofurantetracarboxylic acid dianhydride,5-(2,5-dioxotetrahydrofuranyl)-3-methyl-3-cyclohexene-1,2-dicarboxylicacid dianhydride and bicyclo[2,2,2]-oct-7-ene-2,3,5,6-tetracarboxylicacid dianhydride; and an aromatic ring-containing aliphatictetracarboxylic acid dianhydride such as1,3,3a,4,5,9b-hexahydro-2,5-dioxo-3-furanyl)-naphtho[1,2-c]furan-1,3-dione,1,3,3a,4,5,9b-hexahydro-5-methyl-5-(tetrahydro-2,5-dioxo-3-furanyl)-naphtho[1,2-c]furan-1,3-dioneand1,3,3a,4,5,9b-hexahydro-8-methyl-5-(tetrahydro-2,5-dioxo-3-furanyl)-naphtho[,2-c]furan-1,3-dione.

Among these tetracarboxylic acid dianhydrides, an aromatictetracarboxylic acid dianhydride is preferred. Specifically,pyromellitic acid dianhydride, 3,3′,4,4′-biphenyltetracarboxylic aciddianhydride, 2,3,3′,4′-biphenyltetracarboxylic acid dianhydride,3,3′,4,4′-biphenylethertetracarboxylic acid dianhydride and3,3′,4,4′-benzophenonetetracarboxylic acid dianhydride are preferred,pyromellitic acid dianhydride, 3,3′,4,4′-biphenyltetracarboxylic aciddianhydride and 3,3′,4,4′-benzophenonetetracarboxylic acid dianhydrideare more preferred, and 3,3′,4,4′-biphenyltetracarboxylic aciddianhydride is still more preferred.

As the tetracarboxylic acid dianhydride, one tetracarboxylic aciddianhydride may be used alone, or two or more tetracarboxylic aciddianhydrides may be used in combination.

In the case of using two or more tetracarboxylic acid dianhydrides incombination, aromatic tetracarboxylic acid dianhydrides or aliphatictetracarboxylic acids may be used in combination, or an aromatictetracarboxylic acid dianhydride and an aliphatic tetracarboxylic aciddianhydride may be combined.

The diamine compound is a diamine compound having two amino groups inits molecular structure. The diamine compound includes both aromatic andaliphatic compounds and is preferably an aromatic compound. That is, informula (I), the divalent organic group represented by B is preferablyan aromatic organic group.

The diamine compound includes, for example, an aromatic diamine such asp-phenylenediamine, m-phenylenediamine, 4,4′-diaminodiphenylmethane,4,4′-diaminodiphenylethane, 4,4′-diaminodiphenyl ether,4,4′-diaminodiphenyl sulfide, 4,4′-diaminodiphenylsulfone,1,5-diaminonaphthalene, 3,3-dimethyl-4,4′-diaminobiphenyl,5-amino-1-(4′-aminophenyl)-1,3,3-trimethylindane,6-amino-1-(4′-aminophenyl)-1,3,3-trimethylindane,4,4′-diaminobenzanilide, 3,5-diamino-3′-trifluoromethylbenzanilide,3,5-diamino-4′-trifluoromethylbenzanilide, 3,4′-diaminodiphenyl ether,2,7-diaminofluorene, 2,2-bis(4-aminophenyl)hexafluoropropane,4,4′-methylene-bis(2-chloroaniline),2,2′,5,5′-tetrachloro-4,4′-diaminobiphenyl,2,2′-dichloro-4,4′-diamino-5,5′-dimethoxybiphenyl,3,3′-dimethoxy-4,4′-diaminobiphenyl,4,4′-diamino-2,2′-bis(trifluoromethyl)biphenyl,2,2-bis[(4-(4-aminophenoxy)phenyl)]propane,2,2-bis[(4-(4-aminophenoxy)phenyl)]hexafluoropropane,1,4-bis(4-aminophenoxy)benzene, 4,4′-bis(4-aminophenoxy)-biphenyl,1,3′-bis(4-aminophenoxy)benzene, 9,9-bis(4-aminophenyl)fluorene, 4,4′-(pphenyleneisopropylidene)bisaniline,4,4′-(m-phenyleneisopropylidene)bisaniline,2,2′-bis[(4-(4-amino-2-trifluoromethylphenoxy)phenyl)]hexafluoropropaneand 4,4′-bis[4-(4-amino-2-trifluoromethyl)phenoxy]-octafluorobiphenyl;an aromatic diamine having two amino groups bonded to an aromatic ringand having a heteroatom other than nitrogen atom of the amino group,such as diaminotetraphenylthiophene; and an aliphatic diamine and analicyclic diamine, such as 1,1-metaxylylenediamine, 1,3-propanediamine,tetramethylenediamine, pentamethylenediamine, octamethylenediamine,nonamethylenediamine, 4,4-diaminoheptamethylenediamine,1,4-diaminocyclohexane, isophoronediamine,tetrahydrodicyclopentadienylenediamine,hexahydro-4,7-methanoindanylenedimethylenediamine,tricyclo[6,2,1,0^(2,7)]-undecylenedimethydiamine and4,4′-methylenebis(cyclohexylamine).

Among these diamine compounds, an aromatic diamine compound ispreferred. Specifically, p-phenylenediamine, m-phenylenediamine,4,4′-diaminodiphenylmethane, 4,4′-diaminodiphenyl ether,3,4′-diaminodiphenyl ether, 4,4′-diaminodiphenyl sulfide and4,4′-diaminodiphenylsulfone are preferred, and 4,4′-diaminodiphenylether and p-phenylenediamine are more preferred.

As the diamine compound, one compound may be used alone, or two or morecompounds may be used in combination. In the case of using two or morecompounds in combination, aromatic diamine compounds or aliphaticdiamine compounds may be used in combination, or an aromatic diaminecompound and an aliphatic diamine compound may be combined.

The number average molecular weight of the polyimide precursor ispreferably from 1,000 to 150,000, more preferably from 5,000 to 130,000,still more preferably from 10,000 to 100,000.

When the number average molecular weight of the polyimide precursor isin the range above, reduction in the solubility of the polyimideprecursor for a solvent is suppressed, and the film-forming property islikely ensured.

The number average molecular weight of the polyimide precursor ismeasured by the gel permeation chromatography (GPC) method under thefollowing measurement conditions.

Column: Tosoh TSKgelα-M (7.8 mm, I.D×30 cm)

Eluent: DMF (dimethylformamide)/30 mM LiBr and 60 mM phosphoric acid

Flow velocity: 0.6 mL/min

Injection amount: 60 μL

Detector: RI (differential refractive index detector)

The content (concentration) of the polyimide precursor is suitably from0.1 mass % to 40 mass %, preferably from 0.5 mass % to 25 mass %, morepreferably from 1 mass % to 20 mass %, relative to the entire polyimideprecursor solution.

(Organic Amine Compound)

The organic amine compound is a compound forming an amine salt of thepolyimide precursor (a carboxyl group thereof) to increase thesolubility in an aqueous solvent and at the same time, functioning as animidization accelerator. Specifically, the organic amine compound ispreferably an amine compound having a molecular weight of 170 or less.The organic amine compound is preferably a compound except for thediamine compound working out to a raw material of the polyimideprecursor.

The organic amine compound is preferably a water-soluble compound. The“water-soluble” as used herein means that the object substance dissolvesin a concentration of 1 mass % or more in water at 25° C.

The organic amine compound includes a primary amine compound, asecondary amine compound, and a tertiary amine compound.

Among these, the organic amine compound is preferably at least onecompound selected from a secondary amine compound and a tertiary aminecompound (particularly, a tertiary amine compound). When a tertiaryamine compound or a secondary amine compound (particularly, a tertiaryamine compound) is applied as the organic amine compound, it is likelythat the solubility of the polyimide precursor for a solvent isincreased, the film-forming property is enhanced and the storagestability of the polyimide precursor is improved.

The organic amine compound also includes a divalent or higher polyvalentamine compound, in addition to a monovalent amine compound. When adivalent or higher polyvalent amine compound is applied, it is likelythat a pseudo-crosslinked structure is formed between molecules of thepolyimide precursor and the storage stability of the polyimide precursorsolution is improved.

The primary amine compound includes, for example, methylamine,ethylamine, n-propylamine, isopropylamine, 2-ethanolamine, and2-amino-2-methyl-1-propanol.

The secondary amine compound includes, for example, dimethylamine,2-(methylamino)ethanol, 2-(ethylamino)ethanol, and morpholine.

The tertiary amine compound includes, for example,2-dimethylaminoethanol, 2-diethylaminoethanol, 2-dimethylaminopropanol,pyridine, triethylamine, picoline, methylmorpholine, ethylmorpholine,1,2-dimethylimidazole, and 2-ethyl-4-methylimidazole.

As the organic amine compound, in view of film-forming property, anamine compound (particularly, a tertiary amine compound) having anitrogen-containing heterocyclic structure is also preferred. The aminecompound having a nitrogen-containing heterocyclic structure(hereinafter, referred to as “nitrogen-containing heterocyclic aminecompound”) includes, for example, isoquinolines (an amine compoundhaving an isoquinoline structure), pyridines (an amine compound having apyridine structure), pyrimidines (an amine compound having a pyrimidinestructure), pyrazines (an amine compound having a pyrazine structure),piperazines (an amine compound having a piperazine structure), triazines(an amine compound having a triazine structure), imidazoles (an aminecompound having an imidazole structure), morpholines (an amine compoundhaving a morpholine structure), polyaniline, polypyridine, andpolyamine.

In view of film-forming property, the nitrogen-containing heterocyclicamine compound is preferably at least one member selected from the groupconsisting of morpholines, pyridines and imidazoles, more preferably atleast one member selected from the group consisting ofN-methylmorpholine, pyridine and picoline.

Among these, the organic amine compound is preferably a compound havinga boiling temperature of 60° C. or more (preferably from 60° C. to 200°C., more preferably from 70° C. to 150° C.). When the boilingtemperature of the organic amine compound is 60° C. or more, it islikely that volatilization of the organic amine compound from thepolyimide precursor solution during storage is prevented and reductionin the solubility of the polyimide precursor for a solvent issuppressed.

The organic amine compound is preferably contained in an amount of from50 mol % to 500 mol %, more preferably from 80 mol % to 250 mol %, stillmore preferably from 90 mol % to 200 mol %, relative to the carboxylgroup (—COOH) of the polyimide precursor in the polyimide precursorsolution.

When the content of the organic amine compound is in the range above, itis likely that the solubility of the polyimide precursor for a solventis increased, the film-forming property is enhanced, and the storagestability of the polyimide precursor solution is improved.

As the above-described organic amine compound, one compound may be usedalone, or two or more compounds may be used in combination.

(Aqueous Solvent)

The aqueous solvent is an aqueous solvent containing water.Specifically, the aqueous solvent is suitably a solvent containing 50mass % or more of water relative to the entire aqueous solvent. Waterincludes, for example, distilled water, ion-exchanged water,ultrafiltered water, and pure water.

The content of water is preferably from 50 mass % to 100 mass %, morepreferably from 70 mass % to 100 mass %, still more preferably from 80mass % to 100 mass %, relative to the entire aqueous solvent.

In the case where the aqueous solvent contains a solvent other thanwater, the solvent other than water includes, for example, awater-soluble organic solvent and an aprotic polar solvent. In view oftransparency, mechanical strength, etc. of the polyimide molded body,the solvent other than water is preferably a water-soluble organicsolvent. Above all, from the standpoint of improving various propertiesof the polyimide molded body, such as heat resistance, electricalproperty and solvent resistance, in addition to transparency andmechanical strength, it is preferred that the aqueous solvent does notcontain an aprotic polar solvent or even if an aprotic polar solvent iscontained, the amount thereof is small (for example, 40 mass % or less,preferably 30 mass % or less, relative to the entire aqueous solvent).The “water-soluble” as used herein means that the object substancedissolves in a concentration of 1 mass % or more in water at 25° C.

One of the above-described water-soluble organic solvents may be usedalone, or two or more thereof may be used in combination.

The water-soluble ether-based solvent is a water-soluble solvent havingan ether bond per molecule. The water-soluble ether-based solventincludes, for example, tetrahydrofuran (THF), dioxane, trioxane,1,2-dimethoxyethane, diethylene glycol dimethyl ether, and diethyleneglycol diethyl ether. Among these water-soluble ether-based solvents,tetrahydrofuran and dioxane are preferred.

The water-soluble ketone-based solvent is a water-soluble solvent havinga ketone group per molecule. The water-soluble ketone-based solventincludes, for example, acetone, methyl ethyl ketone, and cyclohexanone.Among these water-soluble ketone solvents, acetone is preferred.

The water-soluble alcohol-based solvent is a water-soluble solventhaving an alcoholic hydroxyl group per molecule. The water-solublealcohol-based solvent includes, for example, methanol, ethanol,l-propanol, 2-propanol, tert-butyl alcohol, ethylene glycol, ethyleneglycol monoalkyl ether, propylene glycol, propylene glycol monoalkylether, diethylene glycol, diethylene glycol monoalkyl ether,1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol,2,3-butanediol, 1,5-pentanediol, 2-butene-1,4-diol,2-methyl-2,4-pentanediol, glycerin,2-ethyl-2-hydroxymethyl-1,3-propanediol, and 1,2,6-hexanetriol. Amongthese water-soluble alcohol solvents, methanol, ethanol, 2-propanol,ethylene glycol, ethylene glycol monoalkyl ether, propylene glycol,propylene glycol monoalkyl ether, diethylene glycol, and diethyleneglycol monoalkyl ether are preferred.

In the case of containing, as the aqueous solvent, an aprotic polarsolvent other than water, the aprotic polar solvent used in combinationis a solvent having a boiling temperature of from 150° C. to 300° C. anda dipole moment of from 3.0 D to 5.0 D. The aprotic polar solventspecifically includes, for example, N-methyl-2-pyrrolidone (NMP),N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc),dimethylsulfoxide (DMSO), hexamethylenephosphoramide (HMPA),N-methylcaprolactam, N-acetyl-2-pyrrolidone, N,N-dimethylimidazolidinone(DMI), N,N′-dimethylpropyleneurea, tetramethylurea, trimethyl phosphate,and triethyl phosphate.

In the case of containing, as the aqueous solvent, a solvent other thanwater, the solvent used in combination suitably has a boilingtemperature of 270° C. or less, preferably from 60° C. to 250° C., morepreferably from 80° C. to 230° C. When the boiling temperature of thesolvent used in combination is in the range above, the solvent otherthan water is less likely to remain in the polyimide molded body, and apolyimide molded body having high mechanical strength is easilyobtained.

Here, dissolution of the polyimide precursor in the solvent iscontrolled by the content of water and the kind and amount of theorganic amine compound. With a low content of water, the polyimideprecursor is likely dissolved in the region where the content of theorganic amine compound is small. Conversely, with a high content ofwater, the polyimide precursor is likely dissolved in the region wherethe content of the organic amine compound is large. In addition, whenthe hydrophilicity is high, for example, when the organic amine compoundcontains a hydroxyl group, the polyimide precursor is likely dissolvedin the region where the content of water is high.

Furthermore, a polyimide precursor synthesized with an organic solventsuch as aprotic polar solvent (e.g., N-methylpyrrolidone (NMP)), thenadded to a poor solvent such as water and alcohol, and therebyprecipitated and separated, may be used as the polyimide precursor.

(Other Additives)

In the production method of a porous polyimide film according to thefirst exemplary embodiment of the present invention, the polyimideprecursor solution may contain, for example, a catalyst for acceleratingthe imidization reaction, and a leveling material for improving thefilm-forming quality.

As the catalyst for accelerating the imidization reaction, a dehydratingagent such as acid anhydride, an acid catalyst such as phenolderivative, sulfonic acid derivative and benzoic acid derivative, andthe like may be used.

The polyimide precursor solution may contain, according to the intendeduse of the porous polyimide film, for example, an electricallyconductive material (conductive (for example, a volume resistivity ofless than 10⁷ Ω·cm) or semi-conductive (for example, a volumeresistivity of from 10⁷ Ω·cm to 10¹³ Ω·cm)) added to impart electricalconductivity.

The electrically conductive agent includes, for example, carbon black(e.g., acidic carbon black with pH of 5.0 or less); a metal (e.g.,aluminum, nickel); a metal oxide (e.g., yttrium oxide, tin oxide); andan ionic electrically conductive substance (e.g., potassium titanate,LiCl). One of these electrically conductive materials may be used alone,or two or more thereof may be used in combination.

The polyimide precursor solution may contain, according to the intendeduse of the porous polyimide film, an inorganic particle added to enhancethe mechanical strength. The inorganic particle includes a particulatematerial such as silica powder, alumina powder, barium sulfate powder,titanium oxide powder, mica and talc.

In addition, the polyimide precursor solution may contain LiCoO₂,LiMn₂O, etc. used as an electrode of a lithium ion battery.

(Production Method of Polyimide Precursor Solution)

The production method of the polyimide precursor solution according tothe first exemplary embodiment of the present invention is notparticularly limited but includes, for example, the following productionmethod.

The production method includes, for example, a method of obtaining thepolyimide precursor solution by polymerizing a tetracarboxylic aciddianhydride and a diamine compound in an aqueous solvent in the presenceof an organic amine compound to produce a resin (polyimide precursor).

This method is advantageous in that use of an aqueous solvent affordshigh productivity and since the polyimide precursor solution is producedin a single stage, the process is simplified.

Other examples include a method where a tetracarboxylic acid dianhydrideand a diamine compound are polymerized in an organic solvent such asaprotic polar solvent (e.g., N-methylpyrrolidone (NMP)) to produce aresin (polyimide precursor), the resin is charged into an aqueoussolvent such as water and alcohol to precipitate the resin (polyimideprecursor) and thereafter, the polyimide precursor and an organic aminecompound are dissolved in an aqueous solvent to obtain the polyimideprecursor solution.

<Porous Polyimide Film>

The porous polyimide film in the first exemplary embodiment of thepresent invention is described below.

In the porous polyimide film obtained by the production method of aporous polyimide film according to the first exemplary embodiment of thepresent invention, generation of cracks is suppressed.

(Characteristics of Porous Polyimide Film)

Although not particularly limited to this range, the porous polyimidefilm of the present invention suitably has a porosity of 30% or more.The porosity is preferably 40% or more, more preferably 50% or more. Theupper limit of the porosity is not particularly limited but is suitably90% or less.

The shape of the vacancy is preferably spherical or close to spherical.In addition, the vacancy is preferably in a configuration wherevacancies are connected and continue with each other (see, FIG. 1D, FIG.2D, and FIG. 3C). The vacancy diameter in the portion where vacanciesare connected with each other is suitably, for example, from 1/100 to ½,preferably from 1/50 to ⅓, more preferably from 1/20 to ¼, of themaximum diameter of the vacancy. Specifically, the average value of thevacancy diameter in the portion where vacancies are connected with eachother is suitably from 5 nm to 1,500 nm.

The average value of the vacancy diameter is not particularly limitedbut is preferably from 0.01 μm to 2.5 μm, more preferably from 0.05 μmto 2.0 μm, still more preferably from 0.1 μm to 1.5 μm, yet still morepreferably from 0.15 μm to 1.0 μm.

In the porous polyimide film in the first exemplary embodiment of thepresent invention, the ratio of maximum diameter and minimum diameter ofthe vacancy (ratio of maximum value and minimum value of the vacancydiameter) is from 1 to 2, preferably from 1 to 1.9, more preferably from1 to 1.8. Of this range, a value closer to 1 is still more preferred.Within this range, the variation in vacancy diameter is reduced. Inaddition, when the porous polyimide film in the first exemplaryembodiment of the present invention is applied, for example, to abattery separator of a lithium ion battery, occurrence of turbulence inthe ion flow is inhibited and therefore, the formation of lithiumdendrite is likely suppressed. The “ratio of maximum diameter andminimum diameter of the vacancy” is a ratio represented by a valueobtained by dividing the maximum diameter by the minimum diameter of thevacancy (i.e., maximum value/minimum value of vacancy diameter).

The maximum value, minimum value and average value of the vacancydiameter, the average value of the vacancy diameter in the portion wherevacancies are connected with each other, and the long diameter and shortdiameter of the vacancy are values observed and measured by a scanningelectron microscope (SEM). Specifically, first, a sample for measurementis prepared by cutting out from the porous polyimide film. Observationand measurement of the sample for measurement are performed using animage processing software standardly equipped in VE SEM manufactured byKeyence Corporation. The observation and measurement are performed on100 vacancies for each vacancy portion in the cross-section of thesample for measurement, and the average value, minimum diameter, maximumdiameter and arithmetic mean diameter are determined for each portion.In the case where the shape of the vacancy is not circular, the longestpart is taken as the diameter.

The thickness of the porous polyimide film is not particularly limitedbut is suitably from 15 μm to 500 μm.

(Use of Porous Polyimide Film)

The use to which the porous polyimide film according to the firstexemplary embodiment of the present invention is applied includes, forexample, a battery separator of a lithium battery, etc.; a separator foran electrolytic capacitor, an electrolyte membrane of a fuel cell, etc.;a battery electrode material; a gas or liquid separation membrane; and alow dielectric material.

In the case where the porous polyimide film according to the firstexemplary embodiment of the present invention is applied, for example,to a battery separator, it is thought that the action such assuppressing the variation in ion stream distribution of the lithium ioninhibits the production of lithium dendrite. This is considered to beattributable to the fact that the variation in vacancy shape and vacancydiameter of the porous polyimide film in an exemplary embodiment of thepresent invention is reduced.

In addition, for example, when the porous polyimide film is applied to abattery electrode material, the capacity of the battery is thought toincrease due to an increase in the opportunity of contacting with anelectrolytic solution. This is presumed to occur because the materialfor electrode, such as carbon black, incorporated into the porouspolyimide film is increased in the amount exposed on the vacancy surfaceor film surface.

Furthermore, the porous polyimide film can also be applied as anelectrolyte membrane, for example, by filling the vacancy with an ionicgel, etc. formed from a so-called ionic liquid by gelling. Since theprocess is simplified by the production method in an exemplaryembodiment of the present invention, a lower-cost electrolyte membraneis expected to be obtained.

Next, a second exemplary embodiment of the present invention isdescribed below.

<Porous Polyimide Film>

The porous polyimide film according to the second exemplary embodimentof the present invention contains a polyimide resin and an uncrosslinkedresin except for a polyimide resin, and the vacancy shape is spherical.

Thanks to the configuration above, in the porous polyimide filmaccording to the second exemplary embodiment of the present invention,cracking of the porous polyimide is suppressed. The reason therefor isnot clearly known but is presumed as follows.

A porous polyimide film is sometimes readily subject to volumecontraction by heat. Since the polyimide film is a rigid resin, in thecase of forming a vacancy in a porous polyimide film composed of only apolyimide resin by using, for example, as a template, an inorganicparticle or an uncrosslinked resin particle, a residual stress due tovolume contraction may be likely produced, giving rise to generation ofcracks.

On the other hand, the porous polyimide film in the second exemplaryembodiment of the present invention contains a polyimide resin and anuncrosslinked resin except for a polyimide resin. By virtue ofcontaining an uncrosslinked resin except for a polyimide resin inaddition to a polyimide resin, the uncrosslinked resin except for apolyimide resin is thought to facilitate relaxation of the residualstress due to volume contraction and suppress the generation of cracks.Furthermore, the vacancy is spherical and this is thought to morefacilitate relaxation of the residual stress due to volume contractionand more suppress the generation of cracks.

For these reasons, the porous polyimide film according to the secondexemplary embodiment of the present invention is believed to ensure thatgeneration of cracks in the porous polyimide film is suppressed.

The porous polyimide film having the above-described configuration ispreferably a porous polyimide film obtained by the production method ofa porous polyimide film, including a first step of forming a coatingfilm containing a polyimide precursor solution in which a polyimideprecursor is dissolved in an aqueous solvent, and an uncrosslinked resinparticle incapable of dissolving in the polyimide precursor solution,followed by drying of the coating film to form a coat containing thepolyimide precursor and the uncrosslinked resin particle, and a secondstep of heating the coat to imidize the polyimide precursor and form apolyimide film, the second step including a treatment for removing theuncrosslinked resin particle with an organic solvent capable ofdissolving the uncrosslinked resin particle.

It is presumed that thanks to this production method, an uncrosslinkedresin except for a polyimide resin can be incorporated into the porouspolyimide and generation of cracks is suppressed.

The polyimide film is obtained, for example, by applying a polyimideprecursor solution dissolved in an organic solvent (e.g.,N-methylpyrrolidone (hereinafter, sometimes referred to as “NMP”)) or apolyimide precursor solution dissolved in a high-polarity solvent suchas N,N-dimethylacetamide (hereinafter, sometimes referred to as “DMAc”),and then heating and shaping the coating.

Conventionally, a porous polyimide film is obtained using a polyimideprecursor solution dissolved in an organic solvent. The method forobtaining a porous polyimide film includes, for example, a method ofobtaining a porous polyimide film having formed therein athree-dimensionally ordered array structure (3DOM structure) ofvacancies by using a silica particle layer as a mold, and a method ofproducing a coat by use of a varnish formed of a polyimide precursorsolution having dispersed therein silica particles, firing the coat, andthen removing the silica particles to obtain a porous polyimide film. Inthe porous polyimide film obtained by these methods, cracking readilyoccurs. The reason therefor is considered because the silica particlehardly absorbs volume contraction in the imidization step and in turn, adistortion (residual stress) is likely generated in the film.

Furthermore, a method where a film is shaped using a solution obtainedby dissolving a water-soluble resin such as polyethylene glycol in apolyimide precursor solution, then contacted with a poor solvent such aswater to precipitate a polyamic acid, thereby promoting creation ofpores, and thereafter, subjected to imidization, is also known. In thismethod, precipitation in a porous manner of a polyamic acid resultingfrom displacement of the solvent such as NMP dissolving the polyamicacid by a poor solvent such as water is utilized, and the shape and sizeof the vacancy can be hardly controlled.

On the other hand, the porous polyimide film in the second exemplaryembodiment of the present invention contains an uncrosslinked resinexcept for a polyimide resin. The uncrosslinked resin particle isremoved with an organic solvent in the imidization step of the polyimideprecursor in the process of producing the porous polyimide film andtherefore, it is considered that control of the shape and size of thevacancy is facilitated and the uncrosslinked resin component dissolvedin the organic solvent readily transfers into the polyimide resin. Thus,an imidization-completed porous polyimide film is obtained in the stateof containing an uncrosslinked resin except for a polyimide resin, andthis is presumed to more facilitate relaxation of the residual stress.

In the porous polyimide film obtained by the above-described productionmethod in the second exemplary embodiment of the present invention, thevariation in vacancy shape, vacancy diameter, etc. is likely suppressed.The reason therefor is presumed because an uncrosslinked resin particleis used in the production process and this effectively contributes tothe relaxation of a residual stress in the imidization step of thepolyimide precursor.

Furthermore, in the porous polyimide film obtained by theabove-described production method in the second exemplary embodiment ofthe present invention, the polyimide precursor is dissolved in anaqueous solvent and therefore, the boiling temperature of the polyimideprecursor solution is about 100° C. The solvent rapidly volatilizes asthe coat containing the polyimide precursor and the uncrosslinked resinparticle is heated, and thereafter, an imidization reaction proceeds.Before deformation of the uncrosslinked resin particle in the coatoccurs due to heat, the uncrosslinked resin particle loses fluidity andbecomes insoluble in an organic solvent or water. For this reason, theshape of vacancy is considered to be likely maintained, making it easyto suppress the variation in vacancy shape, vacancy diameter, etc.

Incidentally, in the case of using a silica particle, a chemical such ashydrofluoric acid needs to be used in the treatment for removing thesilica particle. In addition, in the case of producing a silica particlelayer mold, since a silica particle layer is formed, the productivity islow and the cost is high. Furthermore, in the case of using a silicaparticle, it is thought that since a chemical such as hydrofluoric acidis used, an ion is likely to remain as an impurity.

On the other hand, the porous polyimide film obtained by theabove-described production method in the second exemplary embodiment ofthe present invention does not use a silica particle and therefore, theprocess of obtaining the porous polyimide film is simplified. Inaddition, since a hydrofluoric acid is not used for the removal of theuncrosslinked resin particle, an ion is prevented from remaining as animpurity.

The porous polyimide film according to the second exemplary embodimentof the present invention is described below together with the productionmethod thereof.

The polyimide resin contained in the porous polyimide film according tothe second exemplary embodiment of the present invention is obtainedspecifically by polymerizing a tetracarboxylic acid dianhydride and adiamine compound to produce a polyimide precursor, obtaining a solutionof the polyimide precursor, and subjecting it to an imidizationreaction. More specifically, the polyimide resin is obtained by animidization reaction using a polyimide precursor solution in which thepolyimide precursor is dissolved in an aqueous solvent. The method forobtaining the polyimide precursor solution includes, for example, amethod of obtaining a polyimide precursor solution by polymerizing atetracarboxylic acid dianhydride and a diamine compound in an aqueoussolvent in the presence of an organic amine compound to produce a resin(polyimide precursor), but the method is not limited this example. Thepolyimide precursor solution is described later.

In the case of a porous polyimide film obtained by the above-describedproduction method, the uncrosslinked resin except for a polyimide resin,contained in the porous polyimide film according to the second exemplaryembodiment of the present invention, is a remaining resin component ofthe uncrosslinked resin. The uncrosslinked resin except for a polyimideresin may be contained while maintaining the shape of the uncrosslinkedresin particle or may not have the shape of the uncrosslinked resinparticle. That is, the porous polyimide film obtained by theabove-described production method according to an exemplary embodimentof the present invention may be sufficient if it contains the componentof the uncrosslinked resin except for a polyimide resin. The shape ofthe vacancy of the porous polyimide film obtained by the above-describedproduction method is spherical. The uncrosslinked resin particle isdescribed later.

The “incapable of dissolving in” as used in the second exemplaryembodiment of the present invention means that the object substancesubstantially maintains the form of an uncrosslinked resin particle inthe object liquid at 25° C., and encompasses a case where the objectsubstance dissolves in an amount of 3 mass % or less.

(Production Method of Porous Polyimide Film)

First, the method for producing the porous polyimide film according tothe second exemplary embodiment of the present invention is described.

In the drawings referred to in the description of the production method,the same numerical reference is used for the same component part. As tothe numerical reference in each drawing, 11 indicates an uncrosslinkedresin particle, 2 indicates a binder resin, 3 indicates a base plate, 4indicates a release layer, 5 indicates a polyimide precursor solution, 7indicates a vacancy, 61 indicates a coat (polyimide coat) in the processof performing imidization of the polyimide precursor, and 62 indicates aporous polyimide film.

The method for producing the porous polyimide film according to thesecond exemplary embodiment of the present invention is not particularlylimited but includes, for example, a production method having thefollowing first step and second step.

The first step is a step of forming a coating film containing apolyimide precursor solution in which a polyimide precursor is dissolvedin an aqueous solvent, and an uncrosslinked resin particle incapable ofdissolving in the polyimide precursor solution, followed by drying ofthe coating film to form a coat containing the polyimide precursor andthe uncrosslinked resin particle.

The second step is a step of heating the coat to imidize the polyimideprecursor and form a polyimide film, the second step including atreatment for removing the uncrosslinked resin particle with an organicsolvent capable of dissolving the uncrosslinked resin particle.

In the following, the production method depicted in FIG. 4A, FIG. 4B,and FIG. 4C (one example of the production method according to thesecond exemplary embodiment of the present invention) is described, butthe production method is not limited thereto.

(First Step)

In the first step, a polyimide precursor solution where a polyimideprecursor is dissolved in an aqueous solvent, is prepared. The polyimideprecursor solution having dissolved therein a polyimide precursor ispreferably, for example, a polyimide precursor solution having dissolvedtherein a polyimide precursor and an organic amine compound. In thefollowing, a case using a polyimide precursor solution having dissolvedtherein a polyimide precursor and an organic amine compound is describedas an example. Thereafter, a coating film containing the polyimideprecursor solution and an uncrosslinked resin particle incapable ofdissolving in the polyimide precursor solution is formed on a baseplate. The coating film formed on the base plate is then dried to form acoat containing the polyimide precursor and the uncrosslinked resinparticle. In the following description, the uncrosslinked resin particleis an uncrosslinked resin particle composed of an uncrosslinked resinexcept for a polyimide resin.

In the first step, the method for forming, on a base plate, a coatingfilm containing the polyimide precursor solution and an uncrosslinkedresin particle incapable of dissolving in the polyimide precursorsolution includes specifically the following method.

First, an uncrosslinked resin particle dispersion liquid containing anuncrosslinked resin particle incapable of dissolving in the polyimideprecursor solution, an organic solvent incapable of dissolving theuncrosslinked resin particle, and a binder resin capable of dissolvingin the organic solvent is prepared. Next, the uncrosslinked resinparticle dispersion liquid is applied onto a base plate and dried toform an uncrosslinked resin particle layer. In the uncrosslinked resinparticle layer formed on the base plate, for example, adjacentuncrosslinked resin particles are present without dissolving each other,adjacent uncrosslinked resin particles are at the same time bonded toeach other with a binder resin, and a void is formed betweenuncrosslinked resin particles of the uncrosslinked resin particle layer(see, FIG. 4A).

Meanwhile, a polyimide precursor solution where a polyimide precursorand an organic amine compound are dissolved in an aqueous solvent, ispreviously prepared.

The previously prepared polyimide precursor solution is impregnatedbetween uncrosslinked resin particles of the uncrosslinked resinparticle layer formed on the base plate. By impregnating the polyimideprecursor solution between uncrosslinked resin particles of theuncrosslinked resin particle layer, the void formed betweenuncrosslinked resin particles of the uncrosslinked resin particle layeris filled with the polyimide precursor solution. In order to promotefilling, it is also preferable to remove a gas component in the void byreducing the pressure in the state of the polyimide precursor solutionbeing put into contact with the uncrosslinked resin particle.Thereafter, the coating film is dried, whereby a coat containing thepolyimide precursor and the uncrosslinked resin particle is formed onthe base plate (see, FIG. 4B).

The base plate on which the coat containing the polyimide precursor andthe uncrosslinked resin particle is formed, is not particularly limitedand includes, for example, a base plate made of a resin such aspolystyrene and polyethylene terephthalate; a glass-made base plate; aceramic-made base plate; a metal base plate such as iron and stainlesssteel (SUS); and a composite material base plate formed by combiningthese materials. If desired, the base plate may be subjected to arelease treatment with a silicone-based or fluorine-based release agent,etc. to provide a release layer.

The method for producing the uncrosslinked resin particle dispersionliquid is not particularly limited and includes, for example, a methodwhere each of the uncrosslinked resin particle incapable of dissolvingin the polyimide precursor solution, the organic solvent incapable ofdissolving the uncrosslinked resin particle, and the binder resincapable of dissolving in the organic solvent is weighed and these aremixed and stirred to obtain the uncrosslinked resin particle dispersionliquid. As for the uncrosslinked resin particle, a dispersion liquidwhere uncrosslinked resin particles are previously dispersed may beproduced, or a commercial product where uncrosslinked resin particlesare previously dispersed may be prepared. In the case of producing adispersion liquid where uncrosslinked resin particles are previouslydispersed, the dispersibility of the uncrosslinked resin particle may beincreased, for example, by using at least either one of an ionicsurfactant and a nonionic surfactant.

The binder resin may be previously dissolved in the above-describedorganic solvent or may be mixed with the uncrosslinked resin particleand the organic solvent and dissolved.

The ratio (mass ratio) between the uncrosslinked resin particle and thebinder resin in the uncrosslinked resin particle dispersion liquid issuitably uncrosslinked resin particle: binder resin=from 100:0.5 to100:50, preferably from 100:1 to 100:30, more preferably from 100:2 to100:20. Within this range, in the uncrosslinked resin particle layerformed from the uncrosslinked resin particle dispersion liquid, a statewhere the surface of each uncrosslinked resin particle is partially orentirely covered with the binder resin and adjacent uncrosslinked resinparticles are bonded to each other (including a primarily adheringstate; a so-called pseudo-adhesion state), is likely formed, and a voidproducing an air layer state is readily formed between uncrosslinkedresin particles of the uncrosslinked resin particle layer.

The uncrosslinked resin particle is not particularly limited as long asit does not dissolve in the polyimide precursor solution, and includes,for example, an uncrosslinked resin particle obtained bypolycondensation of a polymerizable monomer, such as polyester resin andurethane resin, and an uncrosslinked resin particle obtained by radicalpolymerization of a polymerizable monomer, such as vinyl resin andolefin resin. The uncrosslinked resin particle obtained by radicalpolymerization includes, for example, uncrosslinked resin particles of(meth)acrylic resin, (meth)acrylic acid ester resin,styrene.(meth)acrylic resin, polystyrene resin and polyethylene resin.

In view of removal of the uncrosslinked resin particle performed in thelater-described second step, the uncrosslinked resin particle ispreferably an uncrosslinked resin particle soluble in an organic solventand is preferably an uncrosslinked resin particle soluble in a solventincapable of dissolving the polyimide resin. That is, the uncrosslinkedresin particle except for a polyimide, contained in the porous polyimidefilm, is preferably an uncrosslinked resin particle soluble intetrahydrofuran, toluene, ethyl acetate, acetone, etc.

Among these, the uncrosslinked resin particle is preferably at least onemember selected from the group consisting of (meth)acrylic resin,(meth)acrylic acid ester resin, styrene.(meth)acrylic resin, andpolystyrene resin.

The term “(meth)acrylic” as used in the second exemplary embodiment ofthe present invention means that both “acrylic” and “methacrylic” areencompassed.

In the case where the uncrosslinked resin particle is, for example, avinyl resin particle, the synthesis method thereof is not particularlylimited, and a known polymerization method (a radical polymerizationmethod such as emulsion polymerization, soap-free emulsionpolymerization, suspension polymerization, miniemulsion polymerizationand microemulsion polymerization) may be applied.

For example, in the case of applying an emulsion polymerization methodto the production of the vinyl resin particle, a monomer such asstyrenes and (meth)acrylic acids is added to water having dissolvedtherein a water-soluble polymerization initiator such as potassiumpersulfate and ammonium persulfate, a surfactant such as sodiumdodecylsulfate and diphenyl oxide disulfonates is further added, ifdesired, and the mixture is heated under stirring to performpolymerization, whereby the vinyl resin particle is obtained.

As for the monomer, the vinyl resin includes, for example, a vinyl resinunit obtained by polymerization of a monomer, e.g., styrenestructure-containing styrenes such as styrene, an alkyl-substitutedstyrene (for example, α-methylstyrene, 2-methylstyrene, 3-methylstyrene,4-methylstyrene, 2-ethylstyrene, 3-ethylstyrene and 4-ethylstyrene), ahalogen-substituted styrene (for example, 2-chlorostyrene,3-chlorostyrene and 4-chlorostyrene) and divinylnaphthalene; vinylgroup-containing esters such as methyl (meth)acrylate, ethyl(meth)acrylate, n-propyl (meth)acrylate, n-butyl (meth)acrylate, lauryl(meth)acrylate and 2-ethylhexyl (meth)acrylate; vinyl nitriles such asacrylonitrile and methacrylonitrile; vinyl ethers such as vinyl methylether and vinyl isobutyl ether; vinyl ketones such as vinyl methylketone, vinyl ethyl ketone and vinyl isopropenyl ketone; acids such as(meth)acrylic acid, maleic acid, cinnamic acid, fumaric acid andvinylsulfonic acid; and bases such as ethyleneimine, vinylpyridine andvinylamine.

As another monomer, a monofunctional monomer such as vinyl acetate, abifunctional monomer such as ethylene glycol dimethacrylate, nonanediacrylate and decanediol diacrylate, or a polyfunctional monomer suchas trimethylolpropane triacrylate and trimethylolpropanetrimethacrylate, may be used in combination.

The vinyl resin may be an uncrosslinked resin using such a monomer aloneor an uncrosslinked resin that is a copolymer using two or more of thesemonomers.

In the case where the monomer used for the uncrosslinked resinconstituting the vinyl resin particle contains styrene, the ratio ofstyrene to all monomer components is preferably from 20 mass % to 100mass %, more preferably from 40 mass % to 100 mass %.

The average particle diameter of the uncrosslinked resin particle is notparticularly limited but is suitably, for example, 2.5 μm or less,preferably 2.0 μm or less, more preferably 1.0 μm or less. The lowerlimit is not particularly limited but is suitably 0.001 μm or more,preferably 0.005 μm or more, more preferably 0.01 μm or more.

As for the average particle diameter of the resin particle, a cumulativedistribution for the volume is drawn from the small diameter side withrespect to divided particle size ranges (channels) by using a particlesize distribution obtained by measurement by means of a laserdiffraction particle size distribution measuring apparatus (for example,LA-700, manufactured by Horiba, Ltd.), and the particle diameter at anaccumulation of 50% relative to all particles is measured as the volumeaverage particle diameter D50v.

The uncrosslinked resin particle includes, for example, a polymethylmethacrylate (MB-Series, produced by Sekisui Plastics Co., Ltd.), and a(meth)acrylic acid ester.styrene copolymer (FS-Series, produced byNippon Paint Co., Ltd.).

The binder resin is not particularly limited as long as it dissolves inthe organic solvent and does not dissolve in the polyimide precursorsolution. The binder resin includes, for example, an acetal resin suchas polyvinylbutyral resin; a polyamide resin such as nylon; a polyesterresin such as polyethylene terephthalate and polyethylene naphthalate; apolyolefin resin such as polyethylene and polypropylene; an acrylicresin; a vinyl resin such as polyvinyl chloride resin and polyvinylidenechloride resin; a polyurethane resin; polyvinylpyrrolidone, polyethyleneglycol, and polyvinyl alcohol. A polyvinylacetal resin and an aliphaticpolyamide resin are preferred.

The organic solvent incapable of dissolving the uncrosslinked resinparticle includes, for example, alcohols such as methanol, ethanol andethylene glycol; cellosolves such as ethylene glycol monomethyl ether,hydrocarbons such as hexane; ketones such as acetone; aromatics such astoluene; esters such as ethyl acetate; and nitriles such asacetonitrile.

Among these, from the standpoint of maintaining the shape of theuncrosslinked resin particle, alcohols and cellosolves are preferred,and the binder resin is preferably a resin soluble in alcohols andcellosolves (for example, acetal resin and polyamide resin).

The method for applying the uncrosslinked resin particle dispersionliquid onto a base plate is not particularly limited and includesvarious methods, for example, a spray coating method, a spin coatingmethod, a roll coating method, a bar coating method, a slit die coatingmethod, and an inkjet coating method.

The coating film formed by applying the uncrosslinked resin particledispersion liquid onto the base plate is dried to obtain theuncrosslinked resin particle layer. The drying temperature may be atemperature capable of maintaining the shape of the uncrosslinked resinparticle and bonding uncrosslinked resin particles to each other (forexample, 100° C.).

Subsequently, a coating film containing the polyimide precursor solutionand the uncrosslinked resin particle is formed by impregnating thepreviously prepared polyimide precursor solution between uncrosslinkedresin particles of the uncrosslinked resin particle layer formed above,and the coating film is then dried to form a coat containing thepolyimide precursor and the uncrosslinked resin particle.

The method for impregnating the polyimide precursor solution is notparticularly limited and includes, for example, a method where the baseplate having formed thereon the uncrosslinked resin particle layer isdipped in the polyimide precursor solution, and a method where thepolyimide precursor solution is applied onto the uncrosslinked resinparticle layer formed on the base plate and impregnated betweenparticles of the uncrosslinked resin particle layer.

The method for applying the polyimide precursor solution onto theuncrosslinked resin particle layer formed on the base plate includesvarious methods, for example, a spray coating method, a spin coatingmethod, a roll coating method, a bar coating method, a slit die coatingmethod, and an inkjet coating method. From the standpoint ofimpregnating the polyimide precursor solution between uncrosslinkedresin particles forming the uncrosslinked resin particle layer, a vacuumimpregnation filling method of applying the polyimide precursor solutiononto the uncrosslinked resin particle layer and then reducing thepressure, thereby filling between uncrosslinked resin particles with thepolyimide precursor solution, is preferably employed, because thepolyimide precursor solution is efficiently impregnated into a voidbetween uncrosslinked resin particles.

The method for forming a coating film containing the polyimide precursorsolution and the uncrosslinked resin particle is not limited to themethods above.

For example, the method specifically includes the following method.First, a polyimide precursor solution where a polyimide precursor and anorganic amine compound are dissolved in an aqueous solvent, is prepared.Next, the polyimide precursor solution and an uncrosslinked resinparticle incapable of dissolving in the polyimide precursor solution aremixed to obtain a polyimide precursor solution having dispersed thereinuncrosslinked resin particles (hereinafter, sometimes referred to as“uncrosslinked resin particle-dispersed polyimide precursor solution”).This uncrosslinked resin particle-dispersed polyimide precursor solutionis applied onto the base plate to form a coating film containing thepolyimide precursor solution and the uncrosslinked resin particle.Uncrosslinked resin particles in the coating film are distributed in thestate of aggregation being suppressed (see, FIG. 6A). Thereafter, thecoating film is dried, whereby a coat containing the polyimide precursorand the uncrosslinked resin particle is formed on the base plate.

The method for producing the uncrosslinked resin particle-dispersedpolyimide precursor solution is not particularly limited and includes,for example, a method of mixing the polyimide precursor solution and theuncrosslinked resin particle in a dry state, and a method of mixing thepolyimide precursor solution with a dispersion liquid whereuncrosslinked resin particles are previously dispersed in an aqueoussolvent. As the dispersion liquid where uncrosslinked resin particlesare previously dispersed in an aqueous solvent, an uncrosslinked resinparticle dispersion liquid where uncrosslinked resin particles arepreviously dispersed in an aqueous solvent may be produced, or acommercially available dispersion liquid where uncrosslinked resinparticles are previously dispersed in an aqueous solvent may beprepared. In the case of producing a dispersion liquid whereuncrosslinked resin particles are previously dispersed, thedispersibility of the uncrosslinked resin particle may be increased, forexample, by using at least either one of an ionic surfactant and anonionic surfactant.

In the polyimide precursor solution having dispersed thereinuncrosslinked resin particles, the ratio of the uncrosslinked resinparticle is suitably, in terms of mass ratio assuming that the solidcontent of the polyimide precursor solution is 100, solid content ofpolyimide precursor solution:uncrosslinked resin particle=from 100:20 to100:200, preferably from 100:25 to 100:180, more preferably from 100:30to 100:150.

The method for applying the uncrosslinked resin particle-dispersedpolyimide precursor solution onto the base plate is not particularlylimited and includes various methods, for example, a spray coatingmethod, a spin coating method, a roll coating method, a bar coatingmethod, a slit die coating method, and an inkjet coating method.

The amount of the polyimide precursor solution coated for obtaining thecoating film containing the polyimide precursor solution obtained aboveand the uncrosslinked resin particle is suitably an amount allowing theuncrosslinked resin particle to be exposed on the coating film surface,because the pore area ratio of the porous polyimide film can beincreased. For example, in the case of impregnating the polyimideprecursor solution between uncrosslinked resin particles forming theuncrosslinked resin particle layer, the polyimide precursor solution issuitably impregnated with a thickness less than the thickness of theuncrosslinked resin particle layer.

In the case of forming the uncrosslinked resin particle-dispersedpolyimide precursor solution on the base plate, the solution is suitablyformed after adding the uncrosslinked resin particle in an amountallowing the uncrosslinked resin particle to be exposed on the coatingfilm surface.

After the coating film containing the polyimide precursor solutionobtained by the method above and the uncrosslinked resin particle isformed, the coating film is dried to form a coat containing thepolyimide precursor and the uncrosslinked resin particle. Specifically,the coating film containing the polyimide precursor solution and theuncrosslinked resin particle is dried, for example, by heat drying,natural drying, vacuum drying or other methods to form a coat. Morespecifically, the coat is formed by drying the coating film such thatthe solvent remaining in the coat accounts for 50% or less, preferably30% or less, relative to the solid content of the coat. This coat is ina state of the polyimide precursor being dissolvable in water.

At the time of formation of the coating film, the coating film may beformed with an amount enough to embed the uncrosslinked resin particlein the coating film. In this case, in the first step, a treatment forexposing the uncrosslinked resin particle may be performed in theprocess of drying the obtained coating film to form the coat, so as toprovide a state of the uncrosslinked resin particle being exposed. Thepore area ratio of the porous polyimide film is increased by performingthe treatment for exposing the uncrosslinked resin particle.

The treatment for exposing the uncrosslinked resin particle specificallyincludes the following method.

When the coating film is formed to embed the uncrosslinked resinparticle layer by impregnating the polyimide precursor solution betweenuncrosslinked resin particles forming the uncrosslinked resin particlelayer, the polyimide precursor solution is present in the regionexceeding the thickness of the uncrosslinked resin particle layer (see,FIG. 4B).

In the process of drying the coating film to form the coat containingthe polyimide precursor and the uncrosslinked resin particle afterobtaining the coating film containing the polyimide precursor solutionand the uncrosslinked resin particle, the coat is in a state of thepolyimide precursor being dissolvable in water. The coat in this stateis treated, for example, by wiping or dipping in water, whereby theuncrosslinked resin particle can be exposed. Specifically, the polyimideprecursor solution present in the region exceeding the thickness of theuncrosslinked resin particle layer is subjected to, for example, atreatment for exposing the uncrosslinked resin particle layer by wetwiping, whereby the polyimide precursor solution present in the regionexceeding the thickness of the uncrosslinked resin particle layer isremoved. As a result, the uncrosslinked resin particle present in theregion at the top of the uncrosslinked resin particle layer (that is,the region on the side distant from the base plate of the uncrosslinkedresin particle layer) is exposed on the coat surface (see, FIG. 4C).

In this connection, in the case of forming the coat on the base plate byusing the uncrosslinked resin particle-dispersed polyimide precursorsolution, when a coat having embedded therein the uncrosslinked resinparticle is formed, the same treatment as the above-described treatmentfor exposing the uncrosslinked resin particle can also be employed asthe treatment for exposing the uncrosslinked resin particle embedded inthe coat.

(Second Step)

The second step is a step of heating the coat containing the polyimideprecursor and the uncrosslinked resin particle, obtained in the firststep, to imidize the polyimide precursor and form a polyimide film. Thesecond step includes a treatment for removing the uncrosslinked resinparticle. A porous polyimide film is obtained through the treatment forremoving the uncrosslinked resin particle.

In the second step, the step of forming a polyimide film is specificallyperformed by heating the coat containing the polyimide precursor and theuncrosslinked resin particle, obtained in the first step, therebyallowing imidization to proceed, and further heating the coat to form apolyimide film. As the imidization proceeds and the imidization ratiorises, the coat becomes hardly dissolvable in an organic solvent.

Thereafter, in the second step, a treatment for removing theuncrosslinked resin particle is performed. As for the removal, theuncrosslinked resin particle may be removed in the process of imidizingthe polyimide precursor by heating the coat or may be removed from apolyimide film after the completion of imidization (after imidization).

In the second exemplary embodiment of the present invention, the processof imidizing the polyimide precursor indicates a process of heating thecoat containing the polyimide precursor and the uncrosslinked resinparticle, obtained in the first step, thereby allowing imidization toproceed and producing a state prior to becoming a polyimide film afterthe completion of imidization.

Specifically, the uncrosslinked resin particle is removed from the coatin the process of heating the coating film obtained in the first step,on which the uncrosslinked resin particle is exposed, and therebyimidizing the polyimide precursor (hereinafter, the coat in this stateis sometimes referred to as “polyimide coat”). Alternatively, theuncrosslinked resin particle may be removed from the polyimide filmafter the completion of imidization. As a result, a porous polyimidefilm, from which the uncrosslinked resin particle is removed, isobtained (see, FIG. 4D).

In the process of removing the uncrosslinked resin particle, theuncrosslinked resin component of the uncrosslinked resin particle isincorporated, as an uncrosslinked resin except for a polyimide resin,into the porous polyimide film. Although not shown, the porous polyimidefilm contains an uncrosslinked resin except for a polyimide resin.

In view of removability, etc. of the uncrosslinked resin particle, thetreatment for removing the uncrosslinked resin particle is preferablyperformed when the imidization ratio of the polyimide precursor in thepolyimide coat is 30% or more, in the process of imidizing the polyimideprecursor. When the imidization ratio becomes 30% or more, the coatbecomes hardly dissolvable in an organic solvent.

The treatment for removing the uncrosslinked resin particle is notparticularly limited as long as the porous polyimide film obtainedcontains an uncrosslinked resin. The treatment includes, for example, amethod of removing the uncrosslinked resin particle by heating, a methodof removing the uncrosslinked resin particle with an organic solventcapable of dissolving the uncrosslinked resin particle, and a method ofremoving the uncrosslinked resin particle by decomposition using alaser, etc. Among these, from the standpoint of suppressing generationof cracks in the porous polyimide film, a method of removing theuncrosslinked resin particle with an organic solvent capable ofdissolving the uncrosslinked resin particle is preferred.

For example, in the method of removing the uncrosslinked resin particleby heating, depending on the kind of the uncrosslinked resin particle, adecomposition gas may be generated by heating, and rupture, cracking,etc. may occur in the porous polyimide film due to the decompositiongas. Therefore, in view of suppressing generation of cracks, a method ofremoving the uncrosslinked resin particle with an organic solventcapable of dissolving the uncrosslinked resin particle is preferablyemployed. Incidentally, it is also effective to further perform heatingafter the removal with an organic solvent capable of dissolving theuncrosslinked resin particle and thereby raise the removal ratio.

In the case where the uncrosslinked resin particle is removed by themethod of removing the uncrosslinked resin particle with an organicsolvent capable of dissolving the uncrosslinked resin particle, theuncrosslinked resin component of the uncrosslinked resin particledissolved in the organic solvent may infiltrate the polyimide film inthe process of removing the uncrosslinked resin particle. Therefore, byemploying this method, an uncrosslinked resin except for a polyimideresin can be aggressively incorporated into the porous polyimide filmobtained. From the standpoint of incorporating an uncrosslinked resinexcept for a polyimide resin, it is preferable to employ the method ofremoving the uncrosslinked resin particle with an organic solventcapable of dissolving the uncrosslinked resin particle. Furthermore,from the standpoint of incorporating an uncrosslinked resin except for apolyimide resin, removal of the uncrosslinked resin particle by themethod above is preferably applied to the coat (polyimide coat) in theprocess of imidizing the polyimide precursor. When the uncrosslinkedresin particle is dissolved with a solvent capable of dissolving theuncrosslinked resin particle in the state of a polyimide coat,infiltration into the polyimide film may be more facilitated.

The method of removing the uncrosslinked resin particle with an organicsolvent capable of dissolving the uncrosslinked resin particle includes,for example, a method of bringing the coat into contact with an organicsolvent capable of dissolving the uncrosslinked resin particle (forexample, dipping in the solvent), and thereby dissolving and removingthe uncrosslinked resin particle. Dipping in this state in the solventis preferred in that the dissolution efficiency for the uncrosslinkedresin particle is increased.

The organic solvent capable of dissolving the uncrosslinked resinparticle for removing the uncrosslinked resin particle is notparticularly limited as long as it is an organic solvent incapable ofdissolving the polyimide coat and the imidization-completed polyimidefilm and capable of dissolving the uncrosslinked resin particle. Theorganic solvent includes, for example, ethers such as tetrahydrofuran;aromatics such as toluene; ketones such as acetone; and esters such asethyl acetate.

Among these, ethers such as tetrahydrofuran are preferred, and it ismore preferable to use tetrahydrofuran. In the case where the aqueoussolvent remains at the time of dissolving the uncrosslinked resinparticle, the aqueous solvent may dissolve in the solvent capable ofdissolving the uncrosslinked resin particle and precipitation of thepolyimide precursor may occur to produce a state similar to that in aso-called wet phase transition method, making it difficult to controlthe vacancy diameter. On this account, the uncrosslinked resin particleis preferably dissolved and removed with an organic solvent afterreducing the amount of the remaining aqueous solvent to 20 mass % orless, preferably 10 mass % or less, relative to the mass of thepolyimide precursor.

In the second step, the heating method when heating the coat obtained inthe first step to allow the progression of imidization and therebyobtain a polyimide film is not particularly limited and includes, forexample, a method of heating the coat in two stages. In the case oftwo-stage heating, the heating conditions specifically include thefollowing heating conditions.

As for the heating conditions in the first stage, the temperature ispreferably a temperature capable of maintaining the shape of theuncrosslinked resin particle. Specifically, the temperature is suitably,for example, from 50° C. to 150° C., preferably from 60° C. to 140° C.The heating time is suitably from 10 minutes to 60 minutes. As theheating temperature is higher, the heating time may be shorter.

As for the heating conditions in the second stage, heating is performed,for example, under the conditions of from 150° C. to 400° C. (preferablyfrom 200° C. to 390° C.) and from 20 minutes to 120 minutes. Under theheating conditions in these ranges, the imidization reaction furtherproceeds and a polyimide film can be formed. At the time of heatingreaction, heating is suitably performed by step-by-step raising thetemperature or gradually raising the temperature at a constant rate,before reaching the final temperature of heating.

The heating conditions are not limited to the above-described two-stageheating method and, for example, a method of heating the coat in asingle stage may be employed. In the case of the single-stage heatingmethod, for example, the imidization may be completed only under theheating conditions of the second stage above.

In the case where a treatment for exposing the uncrosslinked resinparticle is not applied in the first step, from the standpoint ofincreasing the pore area ratio, a treatment for exposing theuncrosslinked resin particle is preferably performed in the second stepto produce a state of the uncrosslinked resin particle being exposed. Inthe second step, the treatment for exposing the uncrosslinked resinparticle is preferably performed in the process of performingimidization of the polyimide precursor or after imidization but beforethe treatment for removing the uncrosslinked resin particle.

For example, in the first step, the uncrosslinked resin particle layeris formed on the base plate (see, FIG. 5A), and the polyimide precursorsolution is impregnated between uncrosslinked resin particles of theuncrosslinked resin particle layer to form a coating film in a state ofthe uncrosslinked resin particle being embedded therein (see, FIG. 5B).Thereafter, a coat containing the polyimide precursor and theuncrosslinked resin particle is formed without performing a treatmentfor exposing the uncrosslinked resin particle in the process of dryingthe coating film to form a coat. The coat formed by this method is acoat in a state of the uncrosslinked resin particle layer being embeddedtherein. Before performing the treatment for removing the uncrosslinkedresin particle by heating, the coat is subjected to a treatment forexposing the uncrosslinked resin particle on the polyimide film in theprocess of imidizing the polyimide precursor or after the completion ofimidization (after imidization).

In the second step, the treatment for exposing the uncrosslinked resinparticle includes, for example, a treatment applied when the polyimidecoat is in the following state.

In the case of performing the treatment for exposing the uncrosslinkedresin particle when the imidization ratio of the polyimide precursor inthe polyimide coat is less than 15% (i.e., a state of the polyimide coatbeing dissolvable in water), the treatment for exposing theuncrosslinked resin particle embedded in the polyimide coat includes awiping treatment, a water dipping treatment, etc.

In the case of performing the treatment for exposing the uncrosslinkedresin particle when the imidization ratio of the polyimide precursor inthe polyimide coat is 15% or more (i.e., a state of being hardlydissolvable in an organic solvent) or when the imidization is completedto generate a polyimide film, the method includes a method of exposingthe uncrosslinked resin particle by mechanical cutting with tools suchas sandpaper, and a method of exposing the uncrosslinked resin particleby decomposition using a laser, etc.

For example, in the case of mechanical cutting, part of theuncrosslinked resin particle present in the region at the top of theuncrosslinked resin particle layer embedded in the polyimide coat (i.e.,the region on the side distant from the base plate of the uncrosslinkedresin particle layer) is cut together with the polyimide coat present atthe top of the uncrosslinked resin particle, and the cut uncrosslinkedresin particle is exposed on the surface of the polyimide coat (see,FIG. 5C).

Thereafter, from the polyimide coat on which the uncrosslinked resinparticle is exposed, the uncrosslinked resin particle is removed by theabove-described treatment for removing the uncrosslinked resin particle.As a result, a porous polyimide film, from which the uncrosslinked resinparticle is removed, is obtained (see, FIG. 5D).

In the case of forming a coat on the base plate by using theuncrosslinked resin particle-dispersed polyimide precursor solution, theuncrosslinked resin particle-dispersed polyimide precursor solution isapplied onto the base plate to form a coating film having embeddedtherein the uncrosslinked resin particle (see, FIG. 6A). When a coatcontaining the polyimide precursor and the uncrosslinked resin particleis formed without performing a treatment for exposing the uncrosslinkedresin particle in the process of drying the coating film to form a coat,a coat having embedded therein the uncrosslinked resin particle isformed. The coat (polyimide coat) in the process of performingimidization by heating the coat is in a state of the uncrosslinked resinparticle layer being embedded therein. As the treatment for exposing theuncrosslinked resin particle, which is performed in the second step soas to increase the pore area ratio, the same treatment as theabove-descried treatment for exposing the uncrosslinked resin particlecan be employed. The uncrosslinked resin particle is then cut togetherwith the polyimide coat present at the top of the uncrosslinked resinparticle to expose the uncrosslinked resin particle on the surface ofthe polyimide coat (see, FIG. 6B).

Thereafter, from the polyimide coat on which the uncrosslinked resinparticle is exposed, the uncrosslinked resin particle is removed by theabove-described treatment for removing the uncrosslinked resin particle.As a result, a porous polyimide film, from which the uncrosslinked resinparticle is removed, is obtained (see, FIG. 6C).

In the second step, the base plate used in the first step for formingthe coat thereon may be separated when the coat becomes a dry coat, maybe separated when the polyimide precursor in the polyimide coat becomeshardly dissolvable in an organic solvent, or may be separated when animidization-completed film is generated.

Through these steps, a porous polyimide film containing a polyimideresin and an uncrosslinked resin except for a polyimide resin isobtained. The porous polyimide film may be post-processed according tothe intended use.

[Polyimide Precursor Solution]

The polyimide precursor solution is not particularly limited as long asa porous polyimide film containing an uncrosslinked resin except for apolyimide resin is obtained. From the standpoint of suppressinggeneration of cracks, the polyimide precursor solution is preferably apolyimide precursor solution where a polyimide precursor is dissolved inan aqueous solvent.

Respective components of the polyimide precursor solution for obtainingthe porous polyimide film according to the second exemplary embodimentof the present invention are described below. Here, the components aredescribed by referring, as an example, to a polyimide precursor solutionhaving dissolved therein a polyimide precursor and an organic aminecompound.

(Polyimide Precursor)

The polyimide precursor is same as the polyimide precursor described inthe first exemplary embodiment as above.

Examples and content and the like of the polyimide precursor are same asexamples and content and the like of the polyimide precursor describedin the first exemplary embodiment.

(Organic Amine Compound)

The organic amine compound is same as the organic amine compounddescribed in the first exemplary embodiment as above.

Examples and content and the like of the organic amine compound are sameas examples and content and the like of the organic amine compounddescribed in the first exemplary embodiment.

(Aqueous Solvent)

The aqueous solvent is same as the aqueous solvent described in thefirst exemplary embodiment as above.

Examples and content and the like of the aqueous solvent are same asexamples and content and the like of the aqueous solvent described inthe first exemplary embodiment.

(Other Additives)

The other additives are same as the other additives described in thefirst exemplary embodiment as above.

(Production Method of Polyimide Precursor Solution)

The production method of the polyimide precursor solution according tothe second exemplary embodiment of the present invention is notparticularly limited but includes, for example, the following productionmethod.

The production method includes, for example, a method of obtaining thepolyimide precursor solution by polymerizing a tetracarboxylic aciddianhydride and a diamine compound in an aqueous solvent in the presenceof an organic amine compound to produce a resin (polyimide precursor).

This method is advantageous in that use of an aqueous solvent affordshigh productivity and since the polyimide precursor solution is producedin a single stage, the process is simplified.

Other examples include a method where a tetracarboxylic acid dianhydrideand a diamine compound are polymerized in an organic solvent such asaprotic polar solvent (e.g., N-methylpyrrolidone (NMP)) to produce aresin (polyimide precursor), the resin is charged into an aqueoussolvent such as water and alcohol to precipitate the resin (polyimideprecursor) and thereafter, the polyimide precursor and an organic aminecompound are dissolved in an aqueous solvent to obtain the polyimideprecursor solution.

Although a polyimide precursor solution where a polyimide precursor andan organic amine compound are dissolved in an aqueous solvent isdescribed as an example, but the polyamide precursor solution is notlimited thereto and includes, for example, a polyimide precursorsolution where an organic amine compound is not dissolved. Specifically,the production method thereof includes, for example, a method ofobtaining a polyimide precursor solution by polymerizing atetracarboxylic acid dianhydride and a diamine compound in an aqueousmixed solvent selected from a water-soluble ether-based compound, awater-soluble ketone-based solvent, a water-soluble alcohol-basedsolvent and water (for example, a mixed solution of a water-solubleether-based solvent and water, a mixed solvent of a water-solubleketone-based solvent and water, and a combination with a water-solublealcohol-based solvent), to produce a resin (polyimide precursor).

The porous polyimide film according to the second exemplary embodimentof the present invention is described below.

(Content of Uncrosslinked Resin Except for Polyimide Resin)

The state of presence of the uncrosslinked resin except for a polyimideresin, contained in the porous polyimide film in the second exemplaryembodiment of the present invention, is not particularly limited. Forexample, the uncrosslinked resin may be present at least either in theinside the porous polyimide film or on the surface of the porouspolyimide film (including the surface of vacancy of the porous polyimidefilm).

From the standpoint of suppressing generation of cracks, the content ofthe uncrosslinked resin except for a polyimide resin is preferably from0.1 mass % to 5 mass %, more preferably from 0.2 mass % to 4.8 mass %,still more preferably from 0.3 mass % to 4.6 mass %. By containing theuncrosslinked resin except for a polyimide resin in this range, thesmoothness of the porous polyimide film may be enhanced.

The amount of the uncrosslinked resin except for a polyimide resin,contained in the porous polyimide film, can be measured, for example, bypyrolysis-gas chromatography-mass analysis (GC-MS). The proportion ofthe uncrosslinked resin except for a polyimide resin can be calculatedfrom the peak assigned to the uncrosslinked resin except for a polyimideresin and the area thereof. In addition, the resin being anuncrosslinked resin means that a polyfunctional component other than thepolyimide resin is not detected in the obtained chromatogram. Theuncrosslinked resin component can also be analyzed by liquidchromatography (HPLC), nuclear magnetic resonance (NMR), etc. afterhydrolyzing the polyimide resin.

(Characteristics of Porous Polyimide Film)

The porous polyimide film in the second exemplary embodiment of thepresent invention has a spherical vacancy shape. The “spherical” vacancyshape as used in an exemplary embodiment of the present inventionencompasses both spherical and substantially spherical (a shape close tospherical). Specifically, this means that a vacancy in which the ratioof long diameter and short diameter (long diameter/short diameter) isfrom 1 to 1.5 exists in a proportion of 90% or more. As the proportionof existence of this vacancy is larger, the proportion of the sphericalvacancy increases. The proportion of a vacancy where the ratio of longdiameter and short diameter (long diameter/short diameter) is from 1 to1.5 is preferably from 93% to 100%, more preferably from 95% to 100%. Asthe ratio of long diameter and short diameter approaches 1, the shapebecomes close to true spherical.

In addition, when the porous polyimide film in the second exemplaryembodiment of the present invention is applied, for example, to abattery separator of a lithium ion battery, occurrence of turbulence inthe ion flow is inhibited and therefore, the formation of lithiumdendrite is likely suppressed.

Although not particularly limited to this range, the porous polyimidefilm in the second exemplary embodiment of the present inventionsuitably has a porosity of 30% or more. The porosity is preferably 40%or more, more preferably 50% or more. The upper limit of the porosity isnot particularly limited but is suitably 90% or less.

The vacancy is preferably in a configuration where vacancies areconnected and continue with each other (see, FIG. 4D, FIG. 5D, and FIG.6C). The vacancy diameter in the portion where vacancies are connectedwith each other is suitably, for example, from 1/100 to ½, preferablyfrom 1/50 to ⅓, more preferably from 1/20 to ¼, of the maximum diameterof the vacancy. Specifically, the average value of the vacancy diameterin the portion where vacancies are connected with each other is suitablyfrom 5 nm to 1,500 nm.

The average value of the vacancy diameter is not particularly limitedbut is preferably from 0.01 μm to 2.5 μm, more preferably from 0.05 μmto 2.0 μm, still more preferably from 0.1 μm to 1.5 μm, yet still morepreferably from 0.15 μm to 1.0 μm.

In the porous polyimide film in the second exemplary embodiment of thepresent invention, the ratio of maximum diameter and minimum diameter ofthe vacancy (ratio of maximum value and minimum value of the vacancydiameter) is from 1 to 2, preferably from 1 to 1.9, more preferably from1 to 1.8. Of this range, a value closer to 1 is still more preferred.Within this range, the variation in vacancy diameter is reduced. Inaddition, when the porous polyimide film in the second exemplaryembodiment of the present invention is applied, for example, to abattery separator of a lithium ion battery, occurrence of turbulence inthe ion flow is inhibited and therefore, the formation of lithiumdendrite is likely suppressed.

The “ratio of maximum diameter and minimum diameter of the vacancy” is aratio represented by a value obtained by dividing the maximum diameterby the minimum diameter of the vacancy (i.e., maximum value/minimumvalue of vacancy diameter).

The maximum value, minimum value and average value of the vacancydiameter, the average value of the vacancy diameter in the portion wherevacancies are connected with each other, and the long diameter and shortdiameter of the vacancy are values observed and measured by a scanningelectron microscope (SEM). Specifically, first, a sample for measurementis prepared by cutting out from the porous polyimide film. Observationand measurement of the sample for measurement are performed using animage processing software standardly equipped in VE SEM manufactured byKeyence Corporation. The observation and measurement are performed on100 vacancies for each vacancy portion in the cross-section of thesample for measurement, and the average value, minimum diameter, maximumdiameter and arithmetic mean diameter are determined for each portion.In the case where the shape of the vacancy is not circular, the longestpart is taken as the diameter. In addition, with respect to each vacancyportion above, observation and measurement of long diameter and shortdiameter are performed using an image processing software standardlyequipped in VE SEM manufactured by Keyence Corporation, and the ratio oflong diameter/short diameter is computed.

The thickness of the porous polyimide film is not particularly limitedbut is suitably from 15 μm to 500 μm.

(Use of Porous Polyimide Film)

The use to which the porous polyimide film according to the secondexemplary embodiment of the present invention is applied is same as theuse to which the porous polyimide film according to the first exemplaryembodiment of the present invention is applied.

Next, a third exemplary embodiment of the present invention is describedbelow.

<Resin Particle-Dispersed Polyimide Precursor Solution and ProductionMethod Thereof>

The method for producing a resin particle-dispersed polyimide precursorsolution according to the third exemplary embodiment of the presentinvention is a method of polymerizing a tetracarboxylic acid dianhydrideand a diamine compound in a resin particle dispersion liquid dispersedin an aqueous solvent, in the presence of an organic amine compound toform a polyimide precursor.

Here, the “incapable of dissolving in” as used in the third exemplaryembodiment of the present invention encompasses a case where the objectsubstance dissolves in an amount of 3 mass % or less in the objectliquid at 25° C.

In the method for producing a resin particle-dispersed polyimideprecursor solution according to the third exemplary embodiment of thepresent invention, thanks to the configuration above, the dispersibilityof resin particles is enhanced, compared with a case where the resinparticle-dispersed polyimide precursor solution is formed by mixing aresin particle dispersion liquid and a polyimide precursor solution. Thereason therefor is not clearly known but is presumed as follows.

The polyimide film is obtained, for example, by applying a polyimideprecursor solution dissolved in an organic solvent (e.g.,N-methylpyrrolidone (hereinafter, sometimes referred to as “NMP”)) or apolyimide precursor solution dissolved in a high-polarity solvent suchas N,N-dimethylacetamide (hereinafter, sometimes referred to as “DMAc”),and then heating and shaping the coating.

In the polyimide film, a particle such as inorganic particle or resinparticle is sometimes incorporated according to the purpose. In thiscase, a polyimide precursor solution having mixed therein particles isused. For example, in the case of producing a particle-dispersedpolyimide precursor solution by mixing an inorganic particle with apolyimide precursor solution dissolved in a high-polarity organicsolvent, the inorganic particle shows a low dispersibility in thepolyimide precursor solution.

On the other hand, in the case of mixing a resin particle with apolyimide precursor solution dissolved in a high-polarity organicsolvent, when the resin particle is a general resin particle (forexample, a polystyrene resin particle), the resin particle is sometimesdissolved with the high-polarity organic solvent, and the dispersibilityof the resin particle in the polyimide precursor solution is low. Inaddition, for example, when a resin particle hardly dissolvable in ahigh-polarity organic solvent is produced by emulsion polymerization,etc., displacement by a high-polarity organic solvent is sometimesperformed so as to mix the resin particle with a polyimide precursorsolution dissolved in a high-porality organic solvent. In this case, theresin particle is sometimes taken out from the resin particle dispersionliquid for performing high-polarity organic solvent displacement, andthe resin particles taken out may undergo aggregation, resulting in lowdispersibility.

In contrast, in the production method of a resin particle-dispersedpolyimide precursor solution according to the third exemplary embodimentof the present invention, the dispersibility of resin particles isenhanced thanks to the above-described configuration. This is consideredto be achieved because the polyimide precursor is formed in a resinparticle dispersion liquid having previously dispersed therein resinparticles, in the presence of an organic amine compound, i.e., thepolyimide precursor is formed in the state of resin particles beingdispersed. Furthermore, since an organic amine compound is present inthe aqueous solvent, the polyimide precursor (a carboxyl group thereof)formed is in the state of being converted to an amine salt by theorganic amine compound. As a result, part of the organic amine salt ofthe polyimide precursor is considered to function as a dispersant forthe resin particle, leading to enhanced dispersibility of the resinparticle.

For these reasons, in the production method of a resinparticle-dispersed polyimide precursor solution according to the thirdexemplary embodiment of the present invention, thanks to theabove-described configuration, the dispersibility of the resin particleis presumed to be enhanced, compared with a case where the resinparticle-dispersed polyimide precursor solution is formed by mixing aresin particle dispersion liquid and a polyimide precursor solution.

In addition, in the production method of a resin particle-dispersedpolyimide precursor solution according to the third exemplary embodimentof the present invention, the polyimide precursor is formed in a resinparticle dispersion liquid having previously dispersed therein resinparticles. On this account, the resin particle-dispersed polyimideprecursor solution in an exemplary embodiment of the present inventionis obtained in a single system (for example, in a single vessel)throughout the process from the production of a resin particledispersion liquid to the production of a resin particle-dispersedpolyimide precursor solution and therefore, the process of producing aresin particle-dispersed polyimide precursor solution is simplified.

Incidentally, in the resin particle-dispersed polyimide precursorsolution obtained by the production method of a resin particle-dispersedpolyimide precursor solution according to the third exemplary embodimentof the present invention, the dispersibility of the resin particle isenhanced. Therefore, in the resin particle-containing polyimide filmobtained from this polyimide precursor solution, the variation in resinparticle distribution is likely suppressed.

The porous polyimide film in the third exemplary embodiment of thepresent invention is obtained by a production method including a firststep of forming a coating film by using the resin particle-dispersedpolyimide precursor solution according to an exemplary embodiment of thepresent invention, followed by drying of the coating film to form acoat, and a second step of heating the coat to cause imidization, thesecond step including a treatment for removing the resin particle. Inthe porous polyimide film obtained by this production method, thevariation in vacancy distribution is likely suppressed. In addition, thevariation in vacancy shape, vacancy diameter, etc. is likely suppressed.The reason therefor is presumed as follows.

The resin particle-dispersed polyimide precursor solution in the thirdexemplary embodiment of the present invention is enhanced indispersibility and therefore, in the porous polyimide film afterremoving the resin particle, the variation in vacancy distribution isconsidered to be likely suppressed.

In addition, the variation in vacancy shape, vacancy diameter, etc. isconsidered to be likely suppressed thanks to use of a resin particle.This is thought to occur because the resin particle effectivelycontributes to the relaxation of residual stress in the imidization stepof the polyimide precursor.

Furthermore, the polyimide precursor is dissolved in an aqueous solventand therefore, the boiling temperature of the polyimide precursorsolution is about 100° C. The solvent rapidly volatilizes as the coatcontaining the polyimide precursor and the resin particle is heated, andthereafter, an imidization reaction proceeds. Before deformation of theresin particle in the coat occurs due to heat, the resin particle losesfluidity and becomes insoluble in an organic solvent. For this reason,the shape of vacancy is considered to be likely maintained.

In the porous polyimide film in the third exemplary embodiment of thepresent invention obtained by forming a resin particle-containingpolyimide film by use of the resin particle-dispersed polyimideprecursor solution according to an exemplary embodiment of the presentinvention and removing the resin particle, generation of cracks islikely suppressed. This is presumed to occur because in the productionmethod of a porous polyimide film in an exemplary embodiment of thepresent invention, a resin particle is used and the use thereofeffectively contributes to the relaxation of residual stress in theimidization step of the polyimide precursor.

Incidentally, the method for producing a porous polyimide film includes,for example, a method of producing a coat by using a polyimide precursorsolution having dispersed therein silica particles, firing the coat, andthen removing the silica particle. However, according to this method, achemical such as hydrofluoric acid needs to be used in the treatment forremoving the silica particle. Therefore, in such a production method,the productivity is low, and the cost is high.

Furthermore, in the case of using a silica particle, it is thought thatsince volume contraction is hardly absorbed in the imidization step, thepolyimide film after imidization is prone to generation of cracks. Inaddition, in the case of using a silica particle, it is thought thatsince a chemical such as hydrofluoric acid is used, an ion is likely toremain as an impurity.

On the other hand, the porous polyimide film obtained by theabove-described production method in the third exemplary embodiment ofthe present invention does not use a silica particle and therefore, theprocess of obtaining the porous polyimide film is simplified. Inaddition, since a hydrofluoric acid is not used for the removal of theresin particle, an ion is prevented from remaining as an impurity.

The production method of a resin particle-dispersed polyimide precursorsolution according to the third exemplary embodiment of the presentinvention and the resin particle-dispersed polyimide precursor solutionobtained by the production method are described below.

[Production Method of Resin Particle-Dispersed Polyimide PrecursorSolution]

In the production method of a resin particle-dispersed polyimideprecursor solution in the third exemplary embodiment of the presentinvention, first, a resin particle dispersion liquid where resinparticles are dispersed in an aqueous solvent, is prepared. Then, atetracarboxylic acid dianhydride and a diamine compound are polymerizedin the resin particle dispersion liquid in the presence of an organicamine compound to form a polyimide precursor.

Specifically, the production method includes a step of preparing a resinparticle dispersion liquid where resin particles are dispersed in anaqueous solvent (hereinafter, sometimes referred to as “resin particledispersion liquid preparation step”), and a step of mixing an organicamine compound, a tetracarboxylic acid dianhydride and a diaminecompound with the resin particle dispersion liquid, thereby polymerizinga tetracarboxylic acid dianhydride and a diamine compound to form apolyimide precursor (hereinafter, sometimes referred to as “polyimideprecursor forming step”).

(Resin Particle Dispersion Liquid Preparation Step)

The resin particle dispersion liquid preparation step is notparticularly limited in the method therefor as long as a resin particledispersion liquid where resin particles are dispersed in an aqueoussolvent is obtained.

The method includes, for example, a method where each of the resinparticle incapable of dissolving in the polyimide precursor solution andthe aqueous solvent for the resin particle dispersion liquid is weighedand these are mixed and stirred to obtain the resin particle dispersionliquid. The method for mixing and stirring the resin particle and theaqueous solvent is not particularly limited and includes, for example, amethod of mixing the resin particle while stirring the aqueous solventFrom the standpoint of increasing the dispersibility of the resinparticle, for example, at least either one of an ionic surfactant and anonionic surfactant may be mixed.

The resin particle dispersion liquid may be a resin particle dispersionliquid obtained by granulating the resin particle in the aqueoussolvent. In the case of granulating the resin particle in the aqueoussolvent, a resin particle dispersion liquid formed by polymerizing amonomer component in the aqueous solvent may be produced. In this case,the dispersion liquid may be a dispersion liquid obtained by a knownpolymerization method. For example, in the case where the resin particleis a vinyl resin particle, a known polymerization method (a radicalpolymerization method such as emulsion polymerization, soap-freeemulsion polymerization, suspension polymerization, miniemulsionpolymerization and microemulsion polymerization) may be applied

For example, in the case of applying an emulsion polymerization methodto the production of the vinyl resin particle, a monomer such asstyrenes and (meth)acrylic acids is added to water having dissolvedtherein a water-soluble polymerization initiator such as potassiumpersulfate and ammonium persulfate, a surfactant such as sodiumdodecylsulfate and diphenyl oxide disulfonates is further added, ifdesired, and the mixture is heated under stirring to performpolymerization, whereby the vinyl resin particle is obtained.

The resin particle dispersion liquid forming step is not limited to theabove-described method, and a commercially available resin particledispersion liquid dispersed in an aqueous solvent may be prepared. Inthe case of using a commercially available resin particle dispersionliquid, an operation such as dilution with an aqueous solvent may beperformed according to the purpose. Furthermore, the resin particledispersion liquid dispersed in an organic solvent may be displaced by anaqueous solvent, as long as the displacement does not affect thedispersibility.

(Polyimide Precursor Forming Step)

Next, a tetracarboxylic acid dianhydride and a diamine compound arepolymerized in the resin particle dispersion liquid in the presence ofan organic amine compound to produce a resin (polyimide precursor),whereby a polyimide precursor solution is obtained.

This method is advantageous in that use of an aqueous solvent affordshigh productivity and since the polyimide precursor solution is producedin a single stage, the process is simplified.

Specifically, an organic amine compound, a tetracarboxylic aciddianhydride, and a diamine compound are mixed with the resin particledispersion liquid prepared in the resin particle dispersion liquidpreparation step, and a tetracarboxylic acid dianhydride and a diaminecompound are polymerized in the presence of an organic amine compound toform a polyimide precursor in the resin particle dispersion liquid. Theorder of mixing an organic amine compound, a tetracarboxylic aciddianhydride and a diamine compound with the resin particle dispersionliquid is not particularly limited.

At the time of polymerizing a tetracarboxylic acid dianhydride and adiamine compound in the resin particle dispersion liquid, the polyimideprecursor may be formed by utilizing the aqueous solvent in the resinparticle dispersion liquid. If desired, an aqueous solvent may be newlymixed. In the case of newly mixing an aqueous solvent, the aqueoussolvent may be an aqueous solvent containing a small amount of anaprotic polar solvent. In addition, other additives may be mixedaccording to the purpose.

Through these steps, a resin particle-dispersed polyimide precursorsolution is obtained.

The materials constituting the resin particle-dispersed polyimideprecursor solution are described below.

(Aqueous Solvent)

As for the aqueous solvent, the aqueous solvent in the resin particledispersion liquid, used for the production of the resin particledispersion liquid, may be utilized as it is at the time of polymerizinga tetracarboxylic acid dianhydride and a diamine compound in the resinparticle dispersion liquid. Alternatively, an aqueous solution suitablefor polymerization may be prepared when polymerizing a tetracarboxylicacid dianhydride and a diamine compound.

The aqueous solvent is an aqueous solvent containing water.Specifically, the aqueous solvent is suitably a solvent containing 50mass % or more of water relative to the entire aqueous solvent. Waterincludes, for example, distilled water, ion-exchanged water,ultrafiltered water, and pure water.

The content of water is preferably from 50 mass % to 100 mass %, morepreferably from 70 mass % to 100 mass %, still more preferably from 80mass % to 100 mass %, relative to the entire aqueous solvent.

The aqueous solvent used when producing the resin particle dispersionliquid is an aqueous solvent containing water. Specifically, the aqueoussolvent for the resin particle dispersion liquid is suitably an aqueoussolvent containing 50 mass % or more of water relative to the entireaqueous solvent. Water includes, for example, distilled water,ion-exchanged water, ultrafiltered water, and pure water. In the case ofcontaining a soluble organic solvent other than water, for example, awater-soluble alcohol-based solvent may be used. The “water-soluble” asused herein means that the object substance dissolves in a concentrationof 1 mass % or more in water at 25° C.

In the case where the aqueous solvent contains a solvent other thanwater, the solvent other than water includes, for example, awater-soluble organic solvent and an aprotic polar solvent. In view oftransparency, mechanical strength, etc. of the polyimide molded body,the solvent other than water is preferably a water-soluble organicsolvent. Above all, from the standpoint of improving various propertiesof the polyimide molded body, such as heat resistance, electricalproperty and solvent resistance, in addition to transparency andmechanical strength, it is preferred that the aqueous solvent does notcontain an aprotic polar solvent or even if an aprotic polar solvent iscontained, the amount thereof is small (for example, 40 mass % or less,preferably 30 mass % or less, relative to the entire aqueous solvent).The “water-soluble” as used herein means that the object substancedissolves in a concentration of 1 mass % or more in water at 25° C.

One of the above-described water-soluble organic solvents may be usedalone, or two or more thereof may be used in combination.

The water-soluble ether-based solvent is a water-soluble solvent havingan ether bond per molecule. The water-soluble ether-based solventincludes, for example, tetrahydrofuran (THF), dioxane, trioxane,1,2-dimethoxyethane, diethylene glycol dimethyl ether, and diethyleneglycol diethyl ether. Among these water-soluble ether-based solvents,tetrahydrofuran and dioxane are preferred.

The water-soluble ketone-based solvent is a water-soluble solvent havinga ketone group per molecule. The water-soluble ketone-based solventincludes, for example, acetone, methyl ethyl ketone, and cyclohexanone.Among these water-soluble ketone solvents, acetone is preferred.

The water-soluble alcohol-based solvent is a water-soluble solventhaving an alcoholic hydroxyl group per molecule. The water-solublealcohol-based solvent includes, for example, methanol, ethanol,1-propanol, 2-propanol, tert-butyl alcohol, ethylene glycol, ethyleneglycol monoalkyl ether, propylene glycol, propylene glycol monoalkylether, diethylene glycol, diethylene glycol monoalkyl ether,1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol,2,3-butanediol, 1,5-pentanediol, 2-butene-1,4-diol,2-methyl-2,4-pentanediol, glycerin,2-ethyl-2-hydroxymethyl-1,3-propanediol, and 1,2,6-hexanetriol. Amongthese water-soluble alcohol-based solvents, methanol, ethanol,2-propanol, ethylene glycol, ethylene glycol monoalkyl ether, propyleneglycol, propylene glycol monoalkyl ether, diethylene glycol, anddiethylene glycol monoalkyl ether are preferred.

In the case of containing, as the aqueous solvent, an aprotic polarsolvent other than water, the aprotic polar solvent used in combinationis a solvent having a boiling temperature of from 150° C. to 300° C. anda dipole moment of from 3.0 D to 5.0 D. The aprotic polar solventspecifically includes, for example, N-methyl-2-pyrrolidone (NMP),N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc),dimethylsulfoxide (DMSO), hexamethylenephosphoramide (HMPA),N-methylcaprolactam, N-acetyl-2-pyrrolidone,1,3-dimethyl-2-imidazolidinone (DMI), N,N′-dimethylpropyleneurea,tetramethylurea, trimethyl phosphate, and triethyl phosphate.

In the case of containing, as the aqueous solvent, a solvent other thanwater, the solvent used in combination suitably has a boilingtemperature of 270° C. or less, preferably from 60° C. to 250° C., morepreferably from 80° C. to 230° C. When the boiling temperature of thesolvent used in combination is in the range above, the solvent otherthan water is less likely to remain in the polyimide molded body, and apolyimide molded body having high mechanical strength is easilyobtained.

Here, dissolution of the polyimide precursor in the solvent iscontrolled by the content of water and the kind and amount of theorganic amine compound. With a low content of water, the polyimideprecursor is likely dissolved in the region where the content of theorganic amine compound is small. Conversely, with a high content ofwater, the polyimide precursor is likely dissolved in the region wherethe content of the organic amine compound is large. In addition, whenthe hydrophilicity is high, for example, when the organic amine compoundcontains a hydroxyl group, the polyimide precursor is likely dissolvedin the region where the content of water is high.

(Resin Particle)

The resin particle is not particularly limited as long as it as long asit does not dissolve in the polyimide precursor solution, and includes,for example, a resin particle obtained by polycondensation of apolymerizable monomer, such as polyester resin and urethane resin, and aresin particle obtained by radical polymerization of a polymerizablemonomer, such as vinyl resin, olefin resin and fluororesin. The resinparticle obtained by radical polymerization includes, for example, resinparticles of (meth)acrylic resin, (meth)acrylic acid ester resin,styrene.(meth)acrylic resin, polystyrene resin and polyethylene resin.

Among these, the resin particle is preferably at least one memberselected from the group consisting of (meth)acrylic resin, (meth)acrylicacid ester resin, styrene.(meth)acrylic resin, and polystyrene resin.

The resin particle may or may not be crosslinked. From the standpoint ofeffectively contributing to the relaxation of residual stress in theimidization step of the polyimide precursor, an uncrosslinked resinparticle is preferred. Furthermore, from the standpoint of simplifyingthe process of producing the resin particle-dispersed polyimideprecursor solution, the resin particle dispersion liquid is preferably avinyl resin particle dispersion liquid obtained by emulsionpolymerization.

The term “(meth)acrylic” as used in the third exemplary embodiment ofthe present invention means that both “acrylic” and “methacrylic” areencompassed.

In the case where the resin particle is a vinyl resin particle, theresin particle is obtained by polymerizing a monomer. The monomer of thevinyl resin includes the following monomers. The vinyl resin includes,for example, a vinyl resin unit obtained by polymerization of a monomer,e.g., styrene structure-containing styrenes such as styrene, analkyl-substituted styrene (for example, α-methylstyrene,2-methylstyrene, 3-methylstyrene, 4-methylstyrene, 2-ethylstyrene,3-ethylstyrene and 4-ethylstyrene), a halogen-substituted styrene (forexample, 2-chlorostyrene, 3-chlorostyrene and 4-chlorostyrene) anddivinylnaphthalene; vinyl group-containing esters such as methyl(meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, n-butyl(meth)acrylate, lauryl (meth)acrylate, 2-ethylhexyl (meth)acrylate andtrimethylolpropane trimethacrylate (TMPTMA); vinyl nitriles such asacrylonitrile and methacrylonitrile; vinyl ethers such as vinyl methylether and vinyl isobutyl ether; vinyl ketones such as vinyl methylketone, vinyl ethyl ketone and vinyl isopropenyl ketone; acids such as(meth)acrylic acid, maleic acid, cinnamic acid, fumaric acid andvinylsulfonic acid; and bases such as ethyleneimine, vinylpyridine andvinylamine.

As another monomer, a monofunctional monomer such as vinyl acetate, abifunctional monomer such as ethylene glycol dimethacrylate, nonanediacrylate and decanediol diacrylate, or a polyfunctional monomer suchas trimethylolpropane triacrylate and trimethylolpropanetrimethacrylate, may be used in combination.

The vinyl resin may be a resin using such a monomer alone or a resinthat is a copolymer using two or more of these monomers.

As described above, the resin particle is preferably uncrosslinked, butin the case of crosslinking the resin particle, when a crosslinkingagent is used at least as part of the monomer components, the ratio ofthe crosslinking agent to all monomer components is preferably from 0mass % to 20 mass %, more preferably from 0 mass % to 5 mass %, stillmore preferably 0 mass %.

In the case where the monomer used for the resin constituting the vinylresin particle contains styrene, the ratio of styrene to all monomercomponents is preferably from 20 mass % to 100 mass %, more preferablyfrom 40 mass % to 100 mass %.

(Average Particle Diameter of Resin Particle)

The average particle diameter of the resin particle is not particularlylimited but is suitably, for example, 2.5 μm or less, preferably 2.0 μmor less, more preferably 1.0 μm or less. The lower limit is notparticularly limited but is suitably 0.001 μm or more, preferably 0.005μm or more, more preferably 0.01 μm or more.

As for the average particle diameter of the resin particle, a cumulativedistribution for the volume is drawn from the small diameter side withrespect to divided particle size ranges (channels) by using a particlesize distribution obtained by measurement by means of a laserdiffraction particle size distribution measuring apparatus (for example,LA-700, manufactured by Horiba, Ltd.), and the particle diameter at anaccumulation of 50% relative to all particles is measured as the volumeaverage particle diameter D50v.

The resin particle may be a commercial product. Specifically, thecrosslinked resin particle includes, for example, a crosslinkedpolymethyl methacrylate (MBX-Series, produced by Sekisui Plastics Co.,Ltd.), a crosslinked polystyrene (SBX-Series, produced by SekisuiPlastics Co., Ltd.), and a crosslinked methyl methacrylate-styrenecopolymer resin particle (MSX-Series, produced by Sekisui Plastics Co.,Ltd.).

The uncrosslinked resin particle includes, for example, a polymethylmethacrylate (MB-Series, produced by Sekisui Plastics Co., Ltd.), and a(meth)acrylic acid ester.styrene copolymer (FS-Series, produced byNippon Paint Co., Ltd.).

(Polyimide Precursor)

The polyimide precursor is obtained by polymerizing a tetracarboxylicacid dianhydride and a diamine compound.

The polyimide precursor is same as the polyimide precursor described inthe first exemplary embodiment as above.

Examples and content and the like of the polyimide precursor are same asexamples and content and the like of the polyimide precursor describedin the first exemplary embodiment.

(Organic Amine Compound)

The organic amine compound is same as the organic amine compounddescribed in the first exemplary embodiment as above.

Examples and content and the like of the organic amine compound are sameas examples and content and the like of the organic amine compounddescribed in the first exemplary embodiment.

(Ratio of Resin Particle and Polyimide Precursor)

In the resin particle-dispersed polyimide precursor solution, the ratioof the resin particle and the polyimide precursor is suitably, in termsof mass ratio assuming that the solid content of the polyimide precursorsolution is 100, solid content of polyimide precursor solution: resinparticle=from 100:20 to 100:200, preferably from 100:25 to 100:180, morepreferably from 100:30 to 100:150.

(Other Additives)

The other additives are same as the other additives described in thefirst exemplary embodiment as above.

<Resin Particle-Containing Polyimide Film>

The resin particle-containing polyimide film according to the thirdexemplary embodiment of the present invention is obtained by applyingthe resin particle-dispersed polyimide precursor solution to form acoating film, and then heating the coating film.

The resin particle-containing polyimide film encompasses not only aresin particle-containing polyimide film after the completion ofimidization but also a partially imidized resin particle-containingpolyimide film before the completion of imidization.

Specifically, the resin particle-containing polyimide film according tothe third exemplary embodiment of the present invention is obtained, forexample, by a method including a step of coating the resinparticle-dispersed polyimide precursor solution according to the thirdexemplary embodiment of the present invention to form a coating film(hereinafter, referred to as “coating film forming step”) and a step ofheating the coating film to form a polyimide film (hereinafter, referredto as “heating step”).

(Coating Film Forming Step)

First, the above-described resin particle-dispersed polyimide precursorsolution is prepared. Then, the resin particle-dispersed polyimideprecursor solution is applied onto a base plate to form a coating film.

The base plate includes, for example, a resin-made base plate; aglass-made base plate; a ceramic-made base plate; a metal base plate;and a composite material base plate formed by combining these materials.Incidentally, the base plate may be subjected to a release treatment toprovide a release layer.

The method for applying the resin particle-dispersed polyimide precursorsolution onto a base plate is not particularly limited and includesvarious methods, for example, a spray coating method, a spin coatingmethod, a roll coating method, a bar coating method, a slit die coatingmethod, and an inkjet coating method.

As the base plate, various base plates can be used according to theintended use. Examples thereof include various substrates applied to aliquid crystal device; a semiconductor substrate having formed thereonan integrated circuit, a wiring substrate having formed thereon wiring,a printed board having provided thereon an electronic component andwiring; a substrate for a wire coating material.

(Heating Step)

Next, the coating film obtained in the coating film forming step aboveis subjected to a drying treatment. A coat (a dry coat beforeimidization) is formed by this drying treatment.

As for the heating conditions in the drying treatment, heating isperformed, for example, at a temperature of from 80° C. to 200° C. forfrom 10 minutes to 60 minutes, and as the temperature is higher, theheating time may be shorter. It is also effective to apply hot airduring heating. At the time of heating, the temperature may bestep-by-step raised or may be raised without changing the speed.

Thereafter, the dry coat before imidization is heated to perform animidization treatment, whereby a polyimide resin layer is formed.

As for the heating conditions in the imidization treatment, heating isperformed, for example, at from 150° C. to 400° C. (preferably from 200°C. to 300° C.) for from 20 minutes to 60 minutes, whereby an imidizationreaction is caused to occur and a polyimide film is formed. At the timeof heating reaction, heating is preferably performed by step-by-stepraising the temperature or gradually raising the temperature at aconstant rate, before reaching the final temperature of heating.

Through these steps, a resin particle-containing polyimide film isformed. Then, if desired, the resin particle-containing polyimide filmis taken out from the base plate to obtain a resin particle-containingpolyimide film. The resin particle-containing polyimide film may bepost-processed according to the intended use.

<Production Method of Porous Polyimide Film>

The production method of a porous polyimide film according to the thirdexemplary embodiment of the present invention includes a first step ofapplying the resin particle-dispersed polyimide precursor solutionaccording to the third exemplary embodiment of the present invention toform a coating film, followed by drying of the coating film to form acoat containing the polyimide precursor and the resin particle, and asecond step of heating the coat to imidize the polyimide precursor andform a polyimide film, the second step including a treatment forremoving the resin particle.

The production method of a porous polyimide film according to the thirdexemplary embodiment of the present invention is described below.

In FIG. 7A, FIG. 7B, and FIG. 7C referred to in the description of theproduction method, the same numerical reference is used for the samecomponent part. As to the numerical reference in FIG. 7A, FIG. 7B, andFIG. 7C, 1 indicates a resin particle, 2 indicates a binder resin, 3indicates a base plate, 4 indicates a release layer, 5 indicates apolyimide precursor solution, 7 indicates a vacancy, 61 indicates a coat(polyimide coat) in the process of performing imidization of thepolyimide precursor, and 62 indicates a porous polyimide film.

In the following, the production method depicted in FIG. 7A, FIG. 7B,and FIG. 7C (one example of the production method according to the thirdexemplary embodiment of the present invention) is described, but theproduction method is not limited thereto.

(First Step)

In the first step, the above-described resin particle-dispersedpolyimide precursor solution is prepared. Thereafter, the resinparticle-dispersed polyimide precursor solution is applied onto a baseplate to form a coating film containing a polyimide precursor solutionand a resin particle. The coating film formed on the base plate is thendried to form a coat containing the polyimide precursor and the resinparticle.

In the first step, the method for forming, on a base plate, a coatingfilm containing the polyimide precursor solution and the resin particleincludes the following method, but the method is not limited thereto.

Specifically, first, a resin particle dispersion where resin particlesare dispersed in an aqueous solvent is prepared. Then, an organic aminecompound, a tetracarboxylic acid dianhydride and a diamine compound aremixed with the resin particle dispersion to prepare a resinparticle-dispersed polyimide precursor solution where a tetracarboxylicacid dianhydride and a diamine compound are polymerized to form apolyimide precursor. Next, the resin particle-dispersed polyimideprecursor solution is applied onto a base plate to form a coating filmcontaining the polyimide precursor solution and the resin particle.Resin particles in the coating films are distributed in the state ofaggregation being suppressed (see, FIG. 7A).

The base plate onto which the resin particle-dispersed polyimideprecursor is applied, is not particularly limited and includes, forexample, a base plate made of a resin such as polystyrene andpolyethylene terephthalate; a glass-made base plate; a ceramic-made baseplate; a metal base plate such as iron and stainless steel (SUS); and acomposite material base plate formed by combining these materials. Ifdesired, the base plate may be subjected to a release treatment with asilicone-based or fluorine-based release agent, etc. to provide arelease layer.

The method for applying the resin particle-dispersed polyimide precursorsolution onto a base plate is not particularly limited and includesvarious methods, for example, a spray coating method, a spin coatingmethod, a roll coating method, a bar coating method, a slit die coatingmethod, and an inkjet coating method.

The amount of the polyimide precursor solution coated for obtaining thecoating film containing the polyimide precursor solution and the resinparticle is suitably an amount allowing the resin particle to be exposedon the coating film surface, because the pore area ratio of the porouspolyimide film can be increased. For example, the coating film issuitably formed with a thickness allowing the resin particle to beexposed on the coating film surface (for example, a coating film wherethe thickness of the coating film is smaller than the particle diameterof the resin particle).

After the coating film containing the polyimide precursor solution andthe resin particle is formed, the coating film is dried to form a coatcontaining the polyimide precursor and the resin particle. Specifically,the coating film containing the polyimide precursor solution and theresin particle is dried, for example, by heat drying, natural drying,vacuum drying or other methods to form a coat. More specifically, thecoat is formed by drying the coating film such that the solventremaining in the coat accounts for 50% or less, preferably 30% or less,relative to the solid content of the coat. This coat is in a state ofthe polyimide precursor being dissolvable in water.

At the time of formation of the coating film, the coating film may beformed with an amount enough to embed the resin particle in the coatingfilm. In this case, a treatment for exposing the resin particle may beperformed in the later-described second step. The pore area ratio of theporous polyimide film is increased by performing the treatment forexposing the resin particle.

(Second Step)

The second step is a step of heating the coat containing the polyimideprecursor and the resin particle, obtained in the first step, to imidizethe polyimide precursor and form a polyimide film. The second stepincludes a treatment for removing the resin particle. A porous polyimidefilm is obtained through the treatment for removing the resin particle.

In the second step, the step of forming a polyimide film is specificallyperformed by heating the coat containing the polyimide precursor and theresin particle, obtained in the first step, thereby allowing imidizationto proceed, and further heating the coat to form a polyimide film. Asthe imidization proceeds and the imidization ratio rises, the coatbecomes hardly dissolvable in an organic solvent.

Thereafter, in the second step, a treatment for removing the resinparticle is performed. As for the removal, the resin particle may beremoved in the process of imidizing the polyimide precursor by heatingthe coat or may be removed from a polyimide film after the completion ofimidization (after imidization).

In an exemplary embodiment of the present invention, the process ofimidizing the polyimide precursor indicates a process of heating thecoat containing the polyimide precursor and the resin particle, obtainedin the first step, thereby allowing imidization to proceed and producinga state prior to becoming a polyimide film after the completion ofimidization.

In view of removability, etc. of the resin particle, the treatment forremoving the resin particle is preferably performed when the imidizationratio of the polyimide precursor in the polyimide coat is 30% or more,in the process of imidizing the polyimide precursor. When theimidization ratio becomes 30% or more, the coat becomes hardlydissolvable in an organic solvent.

The treatment for removing the resin particle includes, for example, amethod of removing the resin particle by heating, a method of removingthe resin particle with an organic solvent capable of dissolving theresin particle, and a method of removing the resin particle bydecomposition using a laser, etc. Among these, a method of removing theresin particle by heating, and a method of removing the resin particlewith an organic solvent capable of dissolving the resin particle arepreferred.

As the method of removing the resin particle by heating, the resinparticle may be decomposed and removed, for example, by the heatingperformed for allowing the imidization to proceed in the process ofimidizing the polyimide precursor. In this case, an operation ofremoving the resin particle with a solvent is omitted, which isadvantageous in view of reducing the number of steps. On the other hand,depending on the kind of the resin particle, a decomposition gas may begenerated by heating, and rupture, cracking, etc. may occur in theporous polyimide film due to the decomposition gas. Therefore, in thiscase, a method of removing the resin particle with an organic solventcapable of dissolving the resin particle is preferably employed.

The method of removing the resin particle with an organic solventcapable of dissolving the resin particle includes, for example, a methodof bringing the coat into contact with an organic solvent capable ofdissolving the resin particle (for example, dipping in the solvent), andthereby dissolving and removing the resin particle. Dipping in thisstate in the solvent is preferred in that the dissolution efficiency forthe resin particle is increased.

The organic solvent capable of dissolving the resin particle forremoving the resin particle is not particularly limited as long as it isan organic solvent incapable of dissolving the polyimide coat and theimidization-completed polyimide film and capable of dissolving the resinparticle. The organic solvent includes, for example, ethers such astetrahydrofuran; aromatics such as toluene; ketones such as acetone; andesters such as ethyl acetate.

In the second step, the heating method when heating the coat obtained inthe first step to allow the progression of imidization and therebyobtain a polyimide film is not particularly limited and includes, forexample, a method of heating the coat in two stages. In the case oftwo-stage heating, the heating conditions specifically include thefollowing heating conditions.

As for the heating conditions in the first stage, the temperature ispreferably a temperature capable of maintaining the shape of the resinparticle. Specifically, the temperature is suitably, for example, from50° C. to 150° C., preferably from 60° C. to 140° C. The heating time issuitably from 10 minutes to 60 minutes. As the heating temperature ishigher, the heating time may be shorter.

As for the heating conditions in the second stage, heating is performed,for example, under the conditions of from 150° C. to 400° C. (preferablyfrom 200° C. to 390° C.) and from 20 minutes to 120 minutes. Under theheating conditions in these ranges, the imidization reaction furtherproceeds and a polyimide film can be formed. At the time of heatingreaction, heating is suitably performed by step-by-step raising thetemperature or gradually raising the temperature at a constant rate,before reaching the final temperature of heating.

The heating conditions are not limited to the above-described two-stageheating method and, for example, a method of heating the coat in asingle stage may be employed. In the case of the single-stage heatingmethod, for example, the imidization may be completed only under theheating conditions of the second stage above.

From the standpoint of increasing the pore area ratio, a treatment forexposing the resin particle is preferably performed in the second stepto produce a state of the resin particle being exposed. In the secondstep, the treatment for exposing the resin particle is preferablyperformed in the process of performing imidization of the polyimideprecursor or after imidization but before the treatment for removing theresin particle.

In this case, for example, when forming a coat on a base plate by usinga resin particle-dispersed polyimide precursor solution, the resinparticle-dispersed polyimide precursor solution is applied onto the baseplate to form a coating film in a state of the resin particle beingembedded therein (see, FIG. 7A). Thereafter, a coat containing thepolyimide precursor and the resin particle is formed by drying thecoating film. The coat formed by this method is in a state of the resinparticle layer being embedded therein. Before performing the treatmentfor removing the resin particle by heating, the coat may be subjected toa treatment for exposing the resin particle on the polyimide film in theprocess of imidizing the polyimide precursor or after the completion ofimidization (after imidization).

In the second step, the treatment for exposing the resin particleincludes, for example, a treatment applied when the polyimide coat is inthe following state.

In the case of performing the treatment for exposing the resin particlewhen the imidization ratio of the polyimide precursor in the polyimidecoat is less than 15% (i.e., a state of the polyimide coat beingdissolvable in water), the treatment for exposing the resin particleembedded in the polyimide coat includes a wiping treatment, a waterdipping treatment, etc.

In the case of performing the treatment for exposing the resin particlewhen the imidization ratio of the polyimide precursor in the polyimidecoat is 15% or more (i.e., a state of being hardly dissolvable in anorganic solvent) or when the imidization is completed to generate apolyimide film, the method includes a method of exposing the resinparticle by mechanical cutting with tools such as sandpaper, and amethod of exposing the resin particle by decomposition using a laser,etc.

For example, in the case of mechanical cutting, part of the resinparticle present in the region at the top of the resin particle embeddedin the polyimide coat (i.e., the region on the side distant from thebase plate of the resin particle) is cut together with the polyimidecoat present at the top of the resin particle, and the cut resinparticle is exposed on the surface of the polyimide coat (see, FIG. 7B).

Thereafter, from the polyimide coat on which the resin particle isexposed, the resin particle is removed by the above-described treatmentfor removing the resin particle. As a result, a porous polyimide film,from which the resin particle is removed, is obtained (see, FIG. 7C).

In the above, the production process of a porous polyimide film, wherethe treatment for exposing the resin particle is applied in the secondstep, is described, but from the standpoint of increasing the pore arearatio, the treatment for exposing the resin particle may be applied inthe first step. In this case, the treatment for exposing the resinparticle may be performed to produce a state of the resin particle beingexposed in the process of drying the coating film to form the coat afterobtaining the coating film in the first step. By performing thistreatment for exposing the resin particle, the pore area ratio of theporous polyimide film is increased.

For example, in the process of drying the coating film to form the coatcontaining the polyimide precursor and the resin particle afterobtaining the coating film containing the polyimide precursor solutionand the resin particle, as described above, the coat is in a state ofthe polyimide precursor being dissolvable in water. The coat in thisstate is treated, for example, by wiping or dipping in water, wherebythe resin particle can be exposed. Specifically, the polyimide precursorsolution present in the region exceeding the thickness of the resinparticle layer is subjected to, for example, a treatment for exposingthe resin particle layer by wet wiping, whereby the polyimide precursorsolution present in the region exceeding the thickness of the resinparticle layer is removed. As a result, the resin particle present inthe region at the top of the resin particle layer (that is, the regionon the side distant from the base plate of the resin particle layer) isexposed on the coat surface.

In the second step, the base plate used in the first step for formingthe coat thereon may be separated when the coat becomes a dry coat, maybe separated when the polyimide precursor in the polyimide coat becomeshardly dissolvable in an organic solvent, or may be separated when animidization-completed film is generated.

Through these steps, a porous polyimide film is obtained. The porouspolyimide film may be post-processed according to the intended use.

<Porous Polyimide Film>

The porous polyimide film in the third exemplary embodiment of thepresent invention is described below.

In the porous polyimide film obtained by the production method of aporous polyimide film according to an exemplary embodiment of thepresent invention, generation of cracks is suppressed.

(Characteristics of Porous Polyimide Film)

Although not particularly limited to this range, the porous polyimidefilm of the present invention suitably has a porosity of 30% or more.The porosity is preferably 40% or more, more preferably 50% or more. Theupper limit of the porosity is not particularly limited but is suitably90% or less.

The shape of the vacancy is preferably spherical or close to spherical.In addition, the vacancy is preferably in a configuration wherevacancies are connected and continue with each other (see, FIG. 7C). Thevacancy diameter in the portion where vacancies are connected with eachother is suitably, for example, from 1/100 to ½, preferably from 1/50 to⅓, more preferably from 1/20 to ¼, of the maximum diameter of thevacancy. Specifically, the average value of the vacancy diameter in theportion where vacancies are connected with each other is suitably from 5nm to 1,500 nm.

The average value of the vacancy diameter is not particularly limitedbut is preferably from 0.01 μm to 2.5 μm, more preferably from 0.05 μmto 2.0 μm, still more preferably from 0.1 μm to 1.5 μm, yet still morepreferably from 0.15 μm to 1.0 μm.

In the porous polyimide film in the third exemplary embodiment of thepresent invention, the ratio of maximum diameter and minimum diameter ofthe vacancy (ratio of maximum value and minimum value of the vacancydiameter) is from 1 to 2, preferably from 1 to 1.9, more preferably from1 to 1.8. Of this range, a value closer to 1 is still more preferred.Within this range, the variation in vacancy diameter is reduced. Inaddition, when the porous polyimide film in the third exemplaryembodiment of the present invention is applied, for example, to abattery separator of a lithium ion battery, occurrence of turbulence inthe ion flow is inhibited and therefore, the formation of lithiumdendrite is likely suppressed. The “ratio of maximum diameter andminimum diameter of the vacancy” is a ratio represented by a valueobtained by dividing the maximum diameter by the minimum diameter of thevacancy (i.e., maximum value/minimum value of vacancy diameter).

The average value of the vacancy diameter and the average value of thevacancy diameter in the portion where vacancies are connected with eachother are values observed and measured by a scanning electron microscope(SEM). Specifically, first, a sample for measurement is prepared bycutting out from the porous polyimide film. Observation and measurementof the sample for measurement are performed using an image processingsoftware standardly equipped in VE SEM manufactured by KeyenceCorporation. The observation and measurement are performed on 100vacancies for each vacancy portion in the cross-section of the samplefor measurement, and the average value, minimum diameter, maximumdiameter and arithmetic mean diameter are determined for each portion.In the case where the shape of the vacancy is not circular, the longestpart is taken as the diameter.

The thickness of the porous polyimide film is not particularly limitedbut is suitably from 15 μm to 500 μm.

(Use of Porous Polyimide Film)

The use to which the porous polyimide film according to the thirdexemplary embodiment of the present invention is applied is same as theuse to which the porous polyimide film according to the first exemplaryembodiment of the present invention is applied.

EXAMPLES

The present invention is described below by referring to Examples, butthe present invention is not limited to these Examples by any means. Inthe following description, unless otherwise indicated, the “parts” and“%” are all on the mass basis.

Examples 1 to 7, Comparative Examples 1 and 2 [Production of PolyimidePrecursor “Water” Solution (PAA-1(a))]

A flask equipped with a stirring bar, a thermometer and a droppingfunnel is filled with 900 g of water. Thereto, 27.28 g (252.27 mmol) ofp-phenylenediamine (molecular weight: 108.14) and 50.00 g (494.32 mmol)of methylmorpholine (organic amine compound) are added and dispersed bystirring at 20° C. for 10 minutes. Furthermore, 72.72 g (247.16 mmol) of3,3′,4,4′-biphenyltetracarboxylic acid dianhydride (molecular weight:294.22) is added to the resulting solution and while keeping thereaction temperature at 20° C., the mixture is stirred for 24 hours toperform dissolution and reaction, whereby Polyimide Precursor “Water”Solution (PAA-1(a)) is obtained.

[Production of Polyimide Precursor “N-Methylpyrrolidone” Solution(RPAA-1(a))]

A flask equipped with a stirring bar, a thermometer and a droppingfunnel is filled with 900 g of N-methylpyrrolidone. Thereto, 27.28 g(252.27 mmol) of p-phenylenediamine is added and dispersed by stirringat 20° C. for 10 minutes. Furthermore, 72.72 g (247.16 mmol) of3,3′,4,4′-biphenyltetracarboxylic acid dianhydride is added to theresulting solution and while keeping the reaction temperature at 20° C.,the mixture is stirred for 24 hours to perform dissolution and reaction,whereby Polyimide Precursor “N-Methylpyrrolidone” Solution (RPAA-1(a))is obtained.

[Production of Polyimide Precursor “Water/Isopropanol” Solution(PAA-2(a))]

500 g of Polyimide Precursor “N-Methylpyrrolidone” Solution (RPAA-1(a))is added dropwise to 3,000 g of water under stirring to precipitate thepolyimide precursor, and 30 g of this polyimide precursor is added to243 g of water and 27 g of isopropanol. Furthermore, 15 g ofmethylmorpholine is added and dissolved under stirring to obtainPolyimide Precursor “Water/Isopropanol” Solution (PAA-2(a)).

[Production of Polyimide Precursor “Water/Isopropanol” Solution(PAA-3(a))]

500 g of Polyimide Precursor “N-Methylpyrrolidone” Solution (RPAA-1(a))is added dropwise to 3,000 g of water under stirring to precipitate thepolyimide precursor, and 30 g of this polyimide precursor is added to243 g of water and 27 g of isopropanol. Furthermore, 15 g of1,2-dimethylimidazole (DMIz) is added and dissolved under stirring toobtain Polyimide Precursor “Water/Isopropanol” Solution (PAA-3(a)).

[Production of Polyimide Precursor “Water/N-Methylpyrrolidone” Solution(PAA-4(a))]

500 g of Polyimide Precursor “N-Methylpyrrolidone” Solution (RPAA-1(a))is added dropwise to 3,000 g of water under stirring to precipitate thepolyimide precursor, and 30 g of this polyimide precursor is added to243 g of water and 27 g of N-methylpyrrolidone. Furthermore, 15 g of1,2-dimethylimidazole is added and dissolved under stirring to obtainPolyimide Precursor “Water/N-Methylpyrrolidone” Solution (PAA-4(a)).

Example 1

10 Parts of an uncrosslinked polymethyl methacrylate.styrene copolymerhaving an average particle diameter of 0.1 μm (FS-102E, produced byNippon Paint Co., Ltd.) and 1 part of polyvinylbutyral resin (S-LECSV-02, produced by Sekisui Chemical Co., Ltd.) are added to 30 parts ofethanol, and the mixture is stirred on a web rotor to produce adispersion liquid. The dispersion liquid is applied onto a glass-madebase plate to have a film thickness of 30 μm after drying, and thecoating formed is dried at 90° C. for 1 hour to form a resin particlelayer.

Polyimide Precursor “Water” Solution (PAA-1(a)) is 10-fold diluted andafter applying Polyimide Precursor “Water” Solution (PAA-1(a)) onto theresin particle layer, vacuum degassing is performed to impregnatePolyimide Precursor “Water” Solution (PAA-1(a)) into a void betweenresin particles. The coating is dried overnight at room temperature (25°C., hereinafter the same) and then wet-wiped to expose the surface ofthe resin particle layer, and the surplus polyimide precursor on theresin particle layer is removed. The resulting coating is heated at 120°C. for 1 hour, then separated from the glass-made base plate, and dippedin toluene to dissolve out the resin particle. After drying, the coatingis subjected to temperature elevation from room temperature to 380° C.at a rate of 10° C./min, held at 380° C. for 1 hour and thereafter,cooled to room temperature to obtain Porous Polyimide Film (PIF-1(a)).

Comparative Example 1

Polyimide Precursor “N-Methylpyrrolidone” Solution (RPAA-1(a)) is10-fold diluted and applied onto the resin particle layer produced inthe same manner as in Example 1, as a result, the resin particle isdissolved. This coating is heated at 120° C. for 1 hour, then separatedfrom the glass-made base plate, and dipped in toluene to dissolve outthe resin particle. After drying, the coating is subjected totemperature elevation from room temperature to 380° C. at a rate of 10°C./min, held at 380° C. for 1 hour and thereafter, cooled to roomtemperature to obtain Porous Polyimide Film (RPIF-1(a)). However, thevacancy diameter ranges from 0.05 μm to 1.01 μm, revealing a widedistribution. This is considered to result because the resin particledissolves and the form of a resin particle cannot be maintained.

Example 2

Porous Polyimide Film (PIF-2(a)) is obtained in the same manner as inExample 1 by 10-fold diluting Polyimide Precursor “Water/Isopropanol”Solution (PAA-2(a)) and applying the solution onto the resin particlelayer produced in the same manner as in Example 1.

Example 3

Polyimide Precursor “Water/Isopropanol” Solution (PAA-2(a)) is 10-folddiluted, applied onto the resin particle layer produced in the samemanner as in Example 1, then dried overnight at room temperature andthereafter, wet-wiped to expose the surface of the resin particle layer,and the surplus polyimide precursor is removed. The resulting coating isheated at 120° C. for 1 hour, separated from the glass-made base plate,then subjected to temperature elevation from room temperature to 380° C.at a rate of 10° C./min, held at 380° C. for 1 hour and thereafter,cooled to room temperature to obtain Porous Polyimide Film (PIF-3(a)).

Example 4

Polyimide Precursor “Water/Isopropanol” Solution (PAA-2(a)) is 10-folddiluted, and an uncrosslinked polymethyl methacrylate.styrene copolymerhaving an average particle diameter of 0.1 μm (FS-102E, produced byNippon Paint Co., Ltd.) is added thereto in a ratio of 10 parts relativeto 10 parts of the polyimide precursor. The mixture is stirred on a webrotor to produce a dispersion liquid, and the dispersion liquid isapplied onto a glass-made base plate to have a film thickness of about30 μm after drying. The coating formed is dried at 90° C. for 1 hour toform a resin particle layer, then subjected to temperature elevationfrom 90° C. to 380° C. at a rate of 10° C./min, held at 380° C. for 1hour and thereafter, cooled to room temperature to obtain PorousPolyimide Film (PIF-4(a)).

Example 5

Polyimide Precursor “Water/Isopropanol” Solution (PAA-2(a)) is 10-folddiluted, and an uncrosslinked polymethyl methacrylate styrene copolymerhaving an average particle diameter of 0.1 μm (FS-102E, produced byNippon Paint Co., Ltd.) is added thereto in a ratio of 10 parts relativeto 10 parts of the polyimide precursor. The mixture is stirred on a webrotor to produce a dispersion liquid, and the dispersion liquid isapplied onto a glass-made base plate to have a film thickness of about30 μm after drying. The coating formed is dried at room temperature for1 hour, stripped from the glass-made base plate, and dipped intetrahydrofuran to dissolve the resin particle. After drying at 90° C.for 1 hour, the coating is subjected to temperature elevation from 90°C. to 380° C. at a rate of 10° C./min, held at 380° C. for 1 hour andthereafter, cooled to room temperature to obtain Porous Polyimide Film(PIF-5(a)).

Example 6

Porous Polyimide Film (PIF-6(a)) is obtained in the same manner as inExample 2 except for using Polyimide Precursor “Water/Isopropanol”Solution (PAA-3(a)).

Example 7

Porous Polyimide Film (PIF-7(a)) is obtained in the same manner as inExample 5 except for using Polyimide Precursor“Water/N-Methylpyrrolidone” Solution (PAA-4(a)). SinceN-methylpyrrolidone has a high boiling temperature and cannot besufficiently removed by drying at room temperature, the vacancy diameteris large compared with the case of using isopropanol.

Comparative Example 2

30 Parts by mass of monodisperse spherical silica particles having anaverage diameter of 550 nm (sphericity: 1.0, particle size distributionindex: 1.20) produced by Nippon Shokubai Co., Ltd. are dispersed in 30parts by mass of N-methylpyrrolidone (NMP), and 20 parts by mass of thesilica particle dispersion liquid is mixed with 100 parts by mass ofPolyimide Precursor “N-Methylpyrrolidone” Solution (RPAA-1(a)). Afterstirring, the resulting solution is applied onto a glass plate, and thecoating is heated at 120° C. for 1 hours, separated from the glass-madebase plate, then subjected to temperature elevation from roomtemperature to 380° C. at a rate of 10° C./min, held at 380° C. for 1hour and thereafter, cooled to room temperature to obtain asilica-polyimide composite film. The silica-polyimide composite film isdipped in 10 mass % hydrogen fluoride water to dissolve and remove thesilica over 6 hours, then thoroughly washed with water and dried toobtain Porous Polyimide Film (RPIF-2(a)).

[Evaluation of Vacancy Diameter Distribution]

The porous polyimide films obtained in Examples 1 to 7 and ComparativeExamples 1 and 2 are evaluated for the vacancy diameter distribution(maximum diameter, minimum diameter, and average diameter).Specifically, the evaluation is performed by the method described above.

[Evaluation of Cracking]

The porous polyimide films obtained in Examples 1 to 7 and ComparativeExamples 1 and 2 are evaluated for cracking. The method therefor isspecifically as follows. A 1 cm²-square area of the polyimide film isexamined through a microscope at a magnification of 500 times, and thepresence or absence of a crack of 0.1 mm or more is observed with aneye.

—Criteria for Evaluation—

A: No crack.

B: From 1 to 3 cracks.

C: 4 or more cracks.

TABLE 1 Polyimide Precursor Porous Tetra- Treatment Treatment Polyimidecarboxylic Di- Amine Resin for for Film Solvent Acid amine CompoundParticle Removal Exposure Example 1 PIF-1(a) water BPDA PDA MMO PMMA/StTol treated Example 2 PIF-2(a) water/IPA BPDA PDA MMO PMMA/St Toltreated Example 3 PIF-3(a) water/IPA BPDA PDA MMO PMMA/St heatingtreated Example 4 PIF-4(a) water/IPA BPDA PDA MMO PMMA/St heating noneExample 5 PIF-5(a) water/IPA BPDA PDA MMO PMMA/St THF none Example 6PIF-6(a) water/IPA BPDA PDA DMIz PMMA/St Tol treated Example 7 PIF-7(a)water/NMP BPDA PDA DMIz PMMA/St Tol none Comparative RPIF-1(a) NMP BPDAPDA — PMMA/St Tol none Example 1 Comparative RPIF-2(a) NMP BPDA PDA —silica hydro- none Example 2 fluoric acid Vacancy Minimum MaximumAverage Porous Polyimide Diameter Diameter Diameter Evaluation of Film(μm) (μm) (μm) Cracking Example 1 PIF-1(a) 0.09 0.12 0.11 A Example 2PIF-2(a) 0.09 0.12 0.11 A Example 3 PIF-3(a) 0.08 0.15 0.13 A Example 4PIF-4(a) 0.06 0.17 0.13 A Example 5 PIF-5(a) 0.08 0.13 0.12 A Example 6PIF-6(a) 0.09 0.12 0.11 A Example 7 PIF-7(a) 0.11 0.18 0.15 AComparative RPIF-1(a) 0.05 1.01 0.65 B Example 1 Comparative RPIF-2(a)0.50 0.59 0.55 C Example 2 Details of abbreviations in Table 1 are asfollows. “PDA”: p-Phenylenediamine “BPDA”:3,3′,4,4′-Biphenyltetracarboxylic acid dianhydride “MMO”:Methylmorpholine “DMIz”: 1,2-Dimethylimidazole “THF”: Tetrahydrofuran“Tol”: Toluene “PMMA/St”: Uncrosslinked polymethyl methacrylate•styrenecopolymer “IPA”: Isopropanol “NMP”: N-Methylpyrrolidone

Examples 1(b) to 5(b), Comparative Examples 1(b) to 3(b) [Production ofPolyimide Precursor “Water” Solution (PAA-1(b))]

A flask equipped with a stirring bar, a thermometer and a droppingfunnel is filled with 900 g of water. Thereto, 27.28 g (252.27 mmol) ofp-phenylenediamine (molecular weight: 108.14) and 50.00 g (494.32 mmol)of methylmorpholine (organic amine compound) are added and dispersed bystirring at 20° C. for 10 minutes. Furthermore, 72.72 g (247.16 mmol) of3,3′,4,4′-biphenyltetracarboxylic acid dianhydride (molecular weight:294.22) is added to the resulting solution and while keeping thereaction temperature at 20° C., the mixture is stirred for 24 hours toperform dissolution and reaction, whereby Polyimide Precursor “Water”Solution (PAA-1(b)) is obtained.

[Production of Polyimide Precursor “N-Methylpyrrolidone” Solution(RPAA-1(b))]

A flask equipped with a stirring bar, a thermometer and a droppingfunnel is filled with 900 g of N-methylpyrrolidone. Thereto, 27.28 g(252.27 mmol) of p-phenylenediamine is added and dispersed by stirringat 20° C. for 10 minutes. Furthermore, 72.72 g (247.16 mmol) of3,3′,4,4′-biphenyltetracarboxylic acid dianhydride is added to theresulting solution and while keeping the reaction temperature at 20° C.,the mixture is stirred for 24 hours to perform dissolution and reaction,whereby Polyimide Precursor “N-Methylpyrrolidone” Solution (RPAA-1(b))is obtained. [Production of Polyimide Precursor “Water/Isopropanol”Solution (PAA-2(b))]500 g of Polyimide Precursor “N-Methylpyrrolidone”Solution (RPAA-1(b)) is added dropwise to 3,000 g of water understirring to precipitate the polyimide precursor, and 30 g of thispolyimide precursor is added to 243 g of water and 27 g of isopropanol.Furthermore, 15 g of methylmorpholine is added and dissolved understirring to obtain Polyimide Precursor “Water/Isopropanol” Solution(PAA-2(b)).

[Production of Polyimide Precursor “Water/Isopropanol” Solution(PAA-3(b))]

500 g of Polyimide Precursor “N-Methylpyrrolidone” Solution (RPAA-1(b))is added dropwise to 3,000 g of water under stirring to precipitate thepolyimide precursor, and 30 g of this polyimide precursor is added to243 g of water and 27 g of isopropanol. Furthermore, 15 g of1,2-dimethylimidazole (DMIz) is added and dissolved under stirring toobtain Polyimide Precursor “Water/Isopropanol” Solution (PAA-3(b)).

[Production of Polyimide Precursor “Water/N-Methylpyrrolidone” Solution(PAA-4(b))]

500 g of Polyimide Precursor “N-Methylpyrrolidone” Solution (RPAA-1(b))is added dropwise to 3,000 g of water under stirring to precipitate thepolyimide precursor, and 30 g of this polyimide precursor is added to243 g of water and 27 g of N-methylpyrrolidone. Furthermore, 15 g of1,2-dimethylimidazole is added and dissolved under stirring to obtainPolyimide Precursor “Water/N-Methylpyrrolidone” Solution (PAA-4(b)).

Example 1(b)

10 Parts of an uncrosslinked polymethyl methacrylate.styrene copolymerhaving an average particle diameter of 0.1 μm (FS-102E, produced byNippon Paint Co., Ltd.) and 1 part of polyvinylbutyral resin (S-LECSV-02, produced by Sekisui Chemical Co., Ltd.) are added to 30 parts ofethanol, and the mixture is stirred on a web rotor to produce adispersion liquid. The dispersion liquid is applied onto a glass-madebase plate to have a film thickness of 30 μm after drying, and thecoating formed is dried at 90° C. for 1 hour to form an uncrosslinkedresin particle layer.

Polyimide Precursor “Water” Solution (PAA-1(b)) is 10-fold diluted andafter applying Polyimide Precursor “Water” Solution (PAA-1(b)) onto theuncrosslinked resin particle layer, vacuum degassing is performed toimpregnate Polyimide Precursor “Water” Solution (PAA-1(b)) into a voidbetween uncrosslinked resin particles. The coating is dried overnight atroom temperature (25° C., hereinafter the same) and then wet-wiped toexpose the surface of the uncrosslinked resin particle layer, and thesurplus polyimide precursor on the uncrosslinked resin particle layer isremoved. The resulting coating is heated at 120° C. for 1 hour, thenseparated from the glass-made base plate, and dipped in tetrahydrofuran(THF) for 30 minutes to dissolve out the uncrosslinked resin particle.After drying, the coating is subjected to temperature elevation fromroom temperature to 380° C. at a rate of 10° C./min, held at 380° C. for1 hour and thereafter, cooled to room temperature to obtain PorousPolyimide Film (PIF-1(b)).

Comparative Example 1(b)

Polyimide Precursor “N-Methylpyrrolidone” Solution (RPAA-1(b)) is10-fold diluted and applied onto the uncrosslinked resin particle layerproduced in the same manner as in Example 1(b), as a result, theuncrosslinked resin particle is dissolved. The coating is heated at 120°C. for 1 hour, then separated from the glass-made base plate, and dippedin THF for 1 hour to dissolve out the uncrosslinked resin particle.After drying, the coating is subjected to temperature elevation fromroom temperature to 380° C. at a rate of 10° C./min, held at 380° C. for1 hour and thereafter, cooled to room temperature to obtain PorousPolyimide Film (RPIF-1(b)). However, the vacancy diameter ranges from0.05 μm to 1.01 μm, revealing a wide distribution. This is considered toresult because the uncrosslinked resin particle dissolves and the formof an uncrosslinked resin particle cannot be maintained. In the PorousPolyimide Film (RPIF-1(b)), the content of the uncrosslinked resincomponent derived from the uncrosslinked resin particle is 0.02%.

Example 2(b)

Porous Polyimide Film (PIF-2(b)) is obtained in the same manner as inExample 1(b) by 10-fold diluting Polyimide Precursor “Water/Isopropanol”Solution (PAA-2(b)) and applying the solution onto the uncrosslinkedresin particle layer produced in the same manner as in Example 1(b).

Example 3(b)

Polyimide Precursor “Water/Isopropanol” Solution (PAA-2(b)) is 10-folddiluted, and an uncrosslinked polymethyl methacrylate.styrene copolymerhaving an average particle diameter of 0.1 μm (FS-102E, produced byNippon Paint Co., Ltd.) is added thereto in a ratio of 10 parts relativeto 10 parts of the polyimide precursor. The mixture is stirred on a webrotor to produce a dispersion liquid, and the dispersion liquid isapplied onto a glass-made base plate to have a film thickness of about30 μm after drying, and the coating formed is dried at room temperaturefor 1 hour, separated from the glass-made base plate, and dipped intetrahydrofuran for 30 minutes. After drying at 90° C. for 1 hour, thecoating is subjected to temperature elevation from 90° C. to 380° C. ata rate of 10° C./min, held at 380° C. for 1 hour and thereafter, cooledto room temperature to obtain Porous Polyimide Film (PIF-3(b)).

Example 4(b)

Porous Polyimide Film (PIF-4(b)) is obtained in the same manner as inExample 2(b) except for using Polyimide Precursor “Water/Isopropanol”Solution (PAA-3(b)).

Example 5(b)

Porous Polyimide Film (PIF-5(b)) is obtained in the same manner as inExample 3(b) except for using Polyimide Precursor“Water/N-Methylpyrrolidone” Solution (PAA-4(b)) and using toluene forthe removal of uncrosslinked resin particle. Since N-methylpyrrolidonehas a high boiling temperature and cannot be sufficiently removed bydrying at room temperature, the vacancy diameter is large compared withthe case of using isopropanol.

Comparative Example 2(b)

30 Parts by mass of monodisperse spherical silica particles having anaverage diameter of 550 nm (sphericity: 1.0, particle size distributionindex: 1.20) produced by Nippon Shokubai Co., Ltd. are dispersed in 30parts by mass of N-methylpyrrolidonc (NMP), and 20 parts by mass of thesilica particle dispersion liquid is mixed with 100 parts by mass ofPolyimide Precursor “N-Methylpyrrolidone” Solution (RPAA-1(b)). Afterstirring, the resulting solution is applied onto a glass plate, and thecoating is heated at 120° C. for 1 hours and then separated from theglass-made base plate. The coating is further subjected to temperatureelevation from room temperature to 380° C. at a rate of 10° C./min, heldat 380° C. for 1 hour and thereafter, cooled to room temperature toobtain a silica-polyimide composite film. The silica-polyimide compositefilm is dipped in 10 mass % hydrogen fluoride water to dissolve andremove the silica over 6 hours, then thoroughly washed with water anddried to obtain Porous Polyimide Film (RPIF-2(b)).

Comparative Example 3(b)

Porous Polyimide Film (RPIF-3(b)) is obtained in the same manner as inExample 3(b) except for using, as the uncrosslinked resin particle, acrosslinked polymethyl methacrylate copolymer having an average particlediameter of 1 μm (SSX-101, produced by Sekisui Plastics Co., Ltd.) andusing toluene as the solvent for the removal of uncrosslinked resinparticle. In the case of using a crosslinked resin particle, the film isa film having many cracks, and this is considered to result because thecrosslinked resin particle swells without dissolving in the solvent.

[Evaluation of Vacancy Diameter Distribution]

The porous polyimide films obtained in Examples 1(b) to 5(b) andComparative Examples 1(b) to 3(b) are evaluated for the vacancy diameterdistribution (maximum diameter, minimum diameter, average diameter, andratio of long diameter and short diameter). Specifically, the evaluationis performed by the method described above.

[Evaluation of Cracking]

The porous polyimide films obtained in Examples 1(b) to 5(b) andComparative Examples 1(b) to 3(b) are evaluated for cracking. The methodtherefor is specifically as follows. A 1 cm²-square area of thepolyimide film is examined through a microscope at a magnification of500 times, and the presence or absence of a crack of 0.1 mm or more isobserved with an eye.

—Criteria for Evaluation—

A: No crack.

B: From 1 to 3 cracks.

C: 4 or more cracks.

[Content of Uncrosslinked Resin Except for Polyimide Resin]

The content of the uncrosslinked resin except for a polyimide resin,contained in the porous polyimide film, is measured by the methoddescribed above.

TABLE 2 Polyimide Precursor Porous Tetra- Treatment Treatment Polyimidecarboxylic Di- Amine Uncrosslinked for for Film Solvent Acid amineCompound Resin Particle Removal Exposure Example 1(b) PIF-1(b) waterBPDA PDA MMO PMMA/St THF treated Example 2(b) PIF-2(b) water/IPA BPDAPDA MMO PMMA/St THF treated Example 3(b) PIF-3(b) water/IPA BPDA PDA MMOPMMA/St THF none Example 4(b) PIF-4(b) water/IPA BPDA PDA DMIz PMMA/StTHF treated Example 5(b) PIF-5(b) water/NMP BPDA PDA DMIz PMMA/St Tolnone Comparative RPIF-1(b) NMP BPDA PDA — PMMA/St THF none Example 1(b)Comparative RPIF-2(b) NMP BPDA PDA — silica hydro- none Example 2(b)fluoric acid Comparative RPIF-3(b) water/IPA BPDA PDA MMO crosslinkedTol none Example 3(b) PMMA Vacancy Proportion of Vacancy with LongContent of Diameter/ Resin Short Except for Porous Minimum MaximumAverage Diameter Evaluation Polyimide Polyimide Diameter DiameterDiameter of 1.5 to 1 of Resin Film (μm) (μm) (μm) (%) Cracking (mass %)Example 1(b) PIF-1(b) 0.09 0.12 0.11 98 A 0.3 Example 2(b) PIF-2(b) 0.090.12 0.11 97 A 0.4 Example 3(b) PIF-3(b) 0.08 0.13 0.12 93 A 3.2 Example4(b) PIF-4(b) 0.09 0.12 0.11 96 A 0.4 Example 5(b) PIF-5(b) 0.11 0.180.15 91 A 4.5 Comparative RPIF-1(b) 0.05 1.01 0.65 5 B 0.02 Example 1(b)Comparative RPIF-2(b) 0.50 0.59 0.55 98 C 0 Example 2(b) ComparativeRPIF-3(b) 0.85 1.85 1.40 75 C 6.5 Example 3(b) Details of abbreviationsin Table 2 are as follows. “PDA”: p-Phenylenediamine “BPDA”:3,3′,4,4′-Biphenyltetracarboxylic acid dianhydride “MMO”;Methylmorpholine “DMIz”: 1,2-Dimethylimidazole “THF”: Tetrahydrofuran“Tol”: Toluene “PMMA/St”: Uncrosslinked polymethyl methacrylate•styrenecopolymer “Crosslinked PMMA”: Crosslinked polymethyl methacrylatecopolymer “IPA”: isopropanol “NMP”: N-Methylpyrrolidone

Examples 1(c) to 6(c), 1A(c) to 10A(c), Reference Example 1(c), andComparative Examples 1(c) to 3(c), 1A(c) to 3A(c) [Preparation of ResinParticle Dispersion Liquid] —Preparation of Resin Particle DispersionLiquid (1)—

770 Parts by mass of styrene, 230 parts by mass of butyl acrylate, 15.7parts by mass of dodecanethiol, 19.8 parts by mass of surfactant Dowfax2A1 (47% solution, produced by Dow Chemical Co.) and 576 parts by massof ion-exchanged water are mixed and emulsified by stirring at arotation speed of 1.500 for 30 minutes to prepare a monomer emulsionliquid. Subsequently, 1.49 parts by mass of Dowfax 2A1 (47% solution,produced by Doe Chemical Co.) and 1,270 parts by mass of ion-exchangedwater are charged into a reaction vessel and heated at 75° C. in anitrogen stream, and a portion (75 parts by mass) of the monomeremulsion liquid is added thereto. A polymerization initiator solutionobtained by dissolving 15 parts by mass of ammonium persulfate in 98parts by mass of ion-exchanged water is then added dropwise over 10minutes, and the reaction is allowed to proceed for 50 minutes after thedropwise addition. Thereafter, the remaining portion of the monomeremulsion liquid is added dropwise over 220 minutes, and the reaction isfurther allowed to proceed for 180 minutes. The reaction solution iscooled to obtain Resin Particle Dispersion Liquid (1) as astyrene.acrylic resin particle dispersion liquid where the solid contentconcentration is adjusted to 30 mass %. The average particle diameter ofthis resin particle is 0.18 μm.

—Resin Particle Dispersion Liquid (2)—

A water dispersion liquid of an uncrosslinked styrene.acryl copolymerhaving an average particle diameter of 0.1 μm (FS-102E, produced byNippon Paint Co., Ltd., solid content concentration: 21 mass %) is usedas Resin Particle Dispersion Liquid (2).

Example 1(c) [Production of Resin Particle-Dispersed Polyimide PrecursorSolution (PAA-1(c))]

After adding 350 g of ion-exchanged water to 100 g in terms of solidcontent of Resin Particle Dispersion Liquid (1), 27.28 g (252.27 mmol)of p-phenylenediamine (molecular weight: 108.14) and 50.00 g (494.32mmol) of methylmorpholine (organic amine compound) are added thereto anddispersed by stirring at 20° C. for 10 minutes. Furthermore, 72.72 g(247.16 mmol) of 3,3′,4,4′-biphenyltetracarboxylic acid dianhydride(molecular weight: 294.22) is added to the resulting solution and whilekeeping the reaction temperature at 40° C., the mixture is stirred for24 hours to perform dissolution and reaction, whereby ResinParticle-Dispersed Polyimide Precursor Solution (PAA-1(c)) is obtained(resin particle/polyimide precursor=100/100 (mass ratio), concentrationof polyimide precursor: 12 mass %).

Example 2(c) [Production of Resin Particle-Dispersed Polyimide PrecursorSolution (PAA-2(c))]

Resin Particle-Dispersed Polyimide Precursor Solution (PAA-2) (resinparticle/polyimide precursor=100/100 (mass ratio), concentration ofpolyimide precursor: 10 mass %) is obtained in the same manner as inExample 1(c) except for changing Resin Particle Dispersion Liquid (1) toResin Particle Dispersion Liquid (2).

Comparative Example 1(c)

[Production of Resin Particle-Dispersed Polyimide Precursor OrganicSolvent Solution (PAA-C1(c))]

After adding 450 g of NMP (N-methylpyrrolidone) to 100 g in terms ofsolid content of an uncrosslinked styrene.acryl copolymer powder havingan average particle diameter of 0.1 μm (FS-102E, produced by NipponPaint Co., Ltd., 27.28 g (252.27 mmol) of p-phenylenediamine (molecularweight: 108.14) is added thereto and dispersed by stirring at 20° C. for10 minutes. Furthermore, 72.72 g (247.16 mmol) of3,3′,4,4′-biphenyltetracarboxylic acid dianhydride (molecular weight:294.22) is added to the resulting solution and while keeping thereaction temperature at 40° C., the mixture is stirred for 24 hours toperform dissolution and reaction, whereby production of ResinParticle-Dispersed Polyimide Precursor Organic Solvent Solution(PAA-C1(c)) is attempted (resin particle/polyimide precursor=100/100(mass ratio), concentration of polyimide precursor: 15 mass %). Allresin particles are dissolved until the polyimide precursor is obtained.

Comparative Example 2(c) [Production of Silica Particle-DispersedPolyimide Precursor Aqueous Solvent Solution (PAA-C2(c))]

After adding 350 g of ion-exchanged water to 100 g in terms of solidcontent of Snowtex (registered trademark) ZL (a water dispersion liquidof silica particle, particle diameter: from 70 nm to 100 nm, produced byNissan Chemical Industries, Ltd.), 27.28 g (252.27 mmol) ofp-phenylenediamine (molecular weight: 108.14) and 50.00 g (494.32 mmol)of methylmorpholine (organic amine compound) are added thereto anddispersed by stirring at 20° C. for 10 minutes. Furthermore, 72.72 g(247.16 mmol) of 3,3′,4,4′-biphenyltetracarboxylic acid dianhydride(molecular weight: 294.22) is added to the resulting solution and whilekeeping the reaction temperature at 40° C., the mixture is stirred for24 hours to perform dissolution and reaction, whereby Particle-DispersedPolyimide Precursor Solution (PAA-C2(c)) having dispersed therein silicaparticles is obtained (silica particle/polyimide precursor=100/100 (massratio), concentration of polyimide precursor: 12 mass %).

Comparative Example 3(c)

[Production of Silica Particle-Dispersed Polyimide Precursor OrganicSolvent Solution (PAA-C3(c))]

30 Parts by mass of monodisperse spherical silica particles having anaverage diameter of 550 nm (sphericity: 1.0, particle size distributionindex: 1.20) produced by Nippon Shokubai Co., Ltd. are dispersed in 30parts by mass of NMP, and 350 g of NMP (N-methylpyrrolidone) is added to100 g in terms of solid content of the silica particle dispersion liquidobtained. Furthermore, 27.28 g (252.27 mmol) of p-phenylenediamine(molecular weight: 108.14) is added thereto and dispersed by stirring at20° C. for 10 minutes. Furthermore, 72.72 g (247.16 mmol) of3,3′,4,4′-biphenyltetracarboxylic acid dianhydride (molecular weight:294.22) is added to the resulting solution and while keeping thereaction temperature at 40° C., the mixture is stirred for 24 hours toperform dissolution and reaction, whereby Particle-Dispersed PolyimidePrecursor Solution (PAA-C3(c)) having dispersed therein silica particlesis obtained (silica particle/polyimide precursor=100/100 (mass ratio),concentration of polyimide precursor: 15 mass %).

Example 3(c) [Production of Resin Particle-Dispersed Polyimide PrecursorSolution (PAA-3(c))]

Resin Particle-Dispersed Polyimide Precursor Solution (PAA-3(c)) (resinparticle/polyimide precursor=100/100 (mass ratio), concentration ofpolyimide precursor: 12 mass %) is obtained in the same manner as inExample 1 except that in Example 1(c), 290 g of ion-exchanged water and60 g of isopropanol are added in place of 350 g of ion-exchanged water.

Example 4(c) [Production of Resin Particle-Dispersed Polyimide PrecursorSolution (PAA-4(c))]

Resin Particle-Dispersed Polyimide Precursor Solution (PAA-4(c)) (resinparticle/polyimide precursor=100/100 (mass ratio), concentration ofpolyimide precursor: 12 mass %) is obtained in the same manner as inExample 3(c) except that in Example 3(c), 47.52 g of1,2-dimethylimidazole is added in place of 50 g of methylmorpholine.

Example 5(c) [Production of Resin Particle-Dispersed Polyimide PrecursorSolution (PAA-5(c))]

Resin Particle-Dispersed Polyimide Precursor Solution (PAA-5(c)) (resinparticle/polyimide precursor=140/100 (mass ratio), concentration ofpolyimide precursor: 10 mass %) is obtained in the same manner as inExample 2(c) except that in Example 2(c), the amount (in terms of solidcontent) of Resin Particle Dispersion Liquid (2) added is changed from100 g to 140 g and the amount of ion-exchanged water added is changedfrom 350 g to 160 g.

Example 6(c) [Production of Resin Particle-Dispersed Polyimide PrecursorSolution (PAA-6(c))]

Resin Particle-Dispersed Polyimide Precursor Solution (PAA-6(c)) (resinparticle/polyimide precursor=100/100 (mass ratio), concentration ofpolyimide precursor: 12 mass %) is obtained in the same manner as inExample 1(c) except that in Example 1(c), 290 g of ion-exchanged waterand 60 g of N-methylpyrrolidone are added in place of 350 g ofion-exchanged water.

Reference Example 1(c) [Production of Resin Particle-Dispersed PolyimidePrecursor Solution (PAA-S1(c))]

A flask equipped with a stirring bar, a thermometer and a droppingfunnel is filled with 900 g of ion-exchanged water. Thereto, 27.28 g(252.27 mmol) of p-phenylenediamine (molecular weight: 108.14) and 50.00g (494.32 mmol) of methylmorpholine (organic amine compound) are addedand dispersed by stirring at 20° C. for 10 minutes. Furthermore, 72.72 g(247.16 mmol) of 3,3′,4,4′-biphenyltetracarboxylic acid dianhydride(molecular weight: 294.22) is added to the resulting solution and whilekeeping the reaction temperature at 40° C., the mixture is stirred for24 hours to perform dissolution and reaction, whereby a polyimideprecursor “water” solution. This polyimide precursor “water” solution is10-fold diluted and after Resin Particle Dispersion Liquid (1) is addedthereto in a ratio of 10 parts as solid content relative to 10 parts ofthe polyimide precursor, the mixture is stirred on a web rotor to obtainResin Particle-Dispersed Polyimide Precursor Solution (PAA-S1(c)).

<Evaluation 1> [Evaluation of Dispersibility]

The resin particle-dispersed polyimide precursor solutions obtained inExamples 1(c) to 6(c), Comparative Examples 1(c) to 3(c) and ReferenceExample 1(c) are evaluated for the dispersibility. The method thereforis specifically as follows.

Each of the resin particle-dispersed polyimide precursor solutionsobtained is stored by leaving the solution to stand still at roomtemperature (25° C.) or under refrigeration (3° C.), and the dispersionstate of resin particles is evaluated with an eye (when the dispersionof resin particles is changed for the worse, layer separation of thesolution or sedimentation of particles can be observed).

—Criteria for Evaluation—

A: The solution is not changed at more than 2 months after initiation ofstorage.

B: Layer separation or sedimentation of particles is observed in morethan 1 month to 2 months after initiation of storage.

C: Layer separation or sedimentation of particles is observed within 1month after initiation of storage

TABLE 3 Particle- Dispersed No. of Evaluation of Polyimide ParticleRatio of Polyimide Precursor Dispersibility Evaluation of PrecursorDispersion Polyimide Tetracarboxylic Amine (room Dispersibility SolutionLiquid Precursor/Particle Solvent Acid Diamine Compound temperature)(refrigeration) Example 1(c) PAA-1(c) (1) 100/100 water BPDA PDA MMO A AExample 2(c) PAA-2(c) (2) 100/100 water BPDA PDA MMO A A Example 3(c)PAA-3(c) (1) 100/100 water/IPA BPDA PDA MMO A A Example 4(c) PAA-4(c)(1) 100/100 water/IPA BPDA PDA DMIz A A Example 5(c) PAA-5(c) (2)100/140 water BPDA PDA MMO A A Example 6(c) PAA-6(c) (1) 100/100 water/BPDA PDA MMO A A NMP Reference PAA-S1(c) (1) 100/100 water BPDA PDA MMOA B Example 1(c) Comparative PAA-C1(c) (2) 100/100 NMP BPDA PDA — cannotbe cannot be Example 1(c) evaluated evaluated Comparative PAA-C2(c)silica (1) 100/100 water BPDA PDA MMO B C Example 2(c) ComparativePAA-C3(c) silica (2) 100/100 NMP BPDA PDA — B C Example 3(c)

<Evaluation 2> [Production of Porous Polyimide Film (PIF-1(c))]

Resin Particle-Dispersed Polyimide Precursor Solution (PAA-1(c)) isapplied onto a glass-made base plate to have a film thickness of about30 μm after drying, and the coating formed is dried at 90° C. for 1hour, then subjected to temperature elevation from 90° C. to 380° C. ata rate of 10° C./min, held at 380° C. for 1 hour and thereafter, cooledto room temperature (25° C., hereinafter the same) to obtain PorousPolyimide Film (PIF-1(c)).

[Production of Porous Polyimide Film (PIF-2(c))]

Resin Particle-Dispersed Polyimide Precursor Solution (PAA-1(c)) isapplied onto a glass-made base plate to have a film thickness of about30 μm after drying, and the coating formed is dried at 90° C. for 1hour, separated from the glass-made base plate, and dipped intetrahydrofuran for 1 hour. The coating is then subjected to temperatureelevation from 90° C. to 380° C. at a rate of 10° C./min, held at 380°C. for 1 hour and thereafter, cooled to room temperature to obtainPorous Polyimide Film (PIF-2(c)).

[Production of Porous Polyimide Films (PIF-3(c)) to (PIF-7(c)),(PIF-9(c)) and (PIF-10(c))]

Porous Polyimide Films (PIF-3(c)) to (PIF-7(c)), (PIF-9(c)) and(PIF-10(c)) are obtained in the same manner as Porous Polyimide Film(PIF-2(c)) except that the kind of the resin particle-dispersedpolyimide precursor solution used and the solvent used for dipping arechanged to those shown in Table 4. In “Treatment for Exposure” of Table4, when treated, a step of exposing the resin particle with sandpaper isadded after separating the coating from the glass-made substrate.

[Production of Porous Polyimide Film (PIF-8(c))]

Porous Polyimide Film (PIF-8(c)) is obtained in the same manner asPorous Polyimide Film (PIF-1(c)) except that the resinparticle-dispersed polyimide precursor solution used is changed to(PAA-2(c)) and a step of applying a treatment for exposing the resinparticle with sandpaper is added after drying.

[Production of Porous Polyimide Film (PIF-C1(c))]

Porous Polyimide Film (PIF-C1(c)) is obtained in the same manner as(PIF-2(c)) except that the solution used is changed to (PAA-C1(c)).

[Production of Porous Polyimide Film (PIF-C2(c))]

(PAA-C2(c)) is applied onto a glass-made base plate to have a filmthickness of about 30 μm after drying, and the coating formed is heatedat 120° C. for 1 hour, separated from the glass-made base plate, thensubjected to temperature elevation from room temperature to 380° C. at arate of 10° C./min, held at 380° C. for 1 hour and thereafter, cooled toroom temperature to obtain a silica-polyimide composite film. Thesilica-polyimide composite film is dipped in 10 mass % hydrogen fluoridewater to dissolve and remove the silica over 6 hours, then thoroughlywashed with water and dried to obtain Porous Polyimide Film (PIF-C2(c)).

[Production of Porous Polyimide Film (PIF-C3(c))]

Porous Polyimide Film (PIF-C3(c)) is obtained in the same manner as inthe production of (PIF-C2(c)) except that the solution used is changedto (PAA-C3(c)).

[Evaluation of Cracking]

The porous polyimide films produced using the resin particle-dispersedpolyimide precursor solutions obtained in Examples 1A(c) to 10A(c) andComparative Examples 1A(c) to 3A(c) are evaluated for cracking. Themethod therefor is specifically as follows. A 1 cm²-square area of thepolyimide film is examined through a microscope at a magnification of500 times, and the presence or absence of a crack of 0.1 mm or more isobserved with an eye.

—Criteria for Evaluation—

A: No crack.

B: From 1 to 3 cracks.

C: 4 or more cracks.

[Evaluation of Vacancy Diameter Distribution]

The porous polyimide films produced using the resin particle-dispersedpolyimide precursor solutions obtained in Examples 1A(c) to 10A(c) andComparative Examples 1A(c) to 3A(c) are evaluated for the vacancydiameter distribution. Specifically, the evaluation is performed by themethod described above.

TABLE 4 Particle- Dispersed No. of Vacancy Polyimide Particle MinimumMaximum Average Precursor Dispersion Porous Treatment for Treatment forDiameter Diameter Diameter Evaluation of Solution Liquid Polyimide FilmRemoval Exposure (μm) (μm) (μm) Cracking Example 1A(c) PAA-1(c) (1)PIF-1(c) heating none 0.17 0.20 0.19 A Example 2A(c) PAA-1(c) (1)PIF-2(c) THF none 0.17 0.20 0.19 A Example 3A(c) PAA-2(c) (2) PIF-3(c)THF treated 0.08 0.13 0.12 A Example 4A(c) PAA-3(c) (1) PIF-4(c) THFtreated 0.09 0.12 0.11 A Example 5A(c) PAA-4(c) (1) PIF-5(c) THF none0.11 0.18 0.15 A Example 6A(c) PAA-5(c) (2) PIF-6(c) THF treated 0.080.12 0.11 A Example 7A(c) PAA-6(c) (1) PIF-7(c) THF treated 0.11 0.180.15 A Example 8A(c) PAA-2(c) (2) PIF-8(c) heating treated 0.08 0.120.11 A Example 9A(c) PAA-1(c) (1) PIF-9(c) THF none 0.18 0.21 0.19 AExample 10A(c) PAA-1(c) (1) PIF-10(c) Tol treated 0.10 0.13 0.12 AComparative PAA-C1(c) (2) PIF-C1(c) THF none 0.05 1.01 0.70 B Example1A(c) Comparative PAA-C2(c) silica (1) PIF-C2(c) hydrofluoric none 0.801.10 1.00 C Example 2A(c) acid Comparative PAA-C3(c) silica (2)PIF-C3(c) hydrofluoric none 0.50 0.59 0.55 C Example 3A(c) acid

In Comparative Example 1A(c), the difference between the maximumdiameter and the minimum diameter (and the ratio of maximum diameter andminimum diameter) is large.

This is presumed to occur because the resin particle is dissolved inN-methylpyrrolidone.

Particles of the particle dispersion liquids in Tables 3 and 4 are asfollows.

(1): Resin Particle Dispersion Liquid (1), average particle diameter:0.18 μm(2): Resin Particle Dispersion Liquid (2), average particle diameter:0.1 μmSilica (1): Snowtex (registered trademark) ZL (particle diameter: from70 nm to 100 nm, produced by Nissan Chemical Industries, Ltd.)Silica (2): Monodisperse spherical silica particle having an averagediameter of 550 nm (sphericity: 1.0, particle size distribution index:1.20, produced by Nippon Shokubai Co., Ltd.)

Details of abbreviations in Tables 3 and 4 are as follows.

“PDA”: p-Phenylenediamine“BPDA”: 3,3′,4,4′-Biphenyltetracarboxylic acid dianhydride

“MMO”: Methylmorpholine “DMIz”: 1,2-Dimethylimidazole “IPA”: Isopropanol“NMP”: N-Methylpyrrolidone “THF”: Tetrahydrofuran “Tol”: Toluene

What is claimed is:
 1. A porous polyimide film comprising a polyimidefilm, an uncrosslinked resin except for a polyimide resin and asubstantially spherical vacancy.
 2. The porous polyimide film as claimedin claim 1, wherein a content of the uncrosslinked resin except for apolyimide resin is from 0.1 mass % to 5 mass % relative to the entireporous polyimide film.
 3. The porous polyimide film as claimed in claim1, wherein the uncrosslinked resin except for a polyimide resin is anuncrosslinked resin soluble in a solvent incapable of dissolving thepolyimide resin.
 4. The porous polyimide film as claimed in claim 1,which is obtained by a production method of a porous polyimide film,comprising: a first step of forming a coating film containing apolyimide precursor solution in which a polyimide precursor is dissolvedin an aqueous solvent, and an uncrosslinked resin particle incapable ofdissolving in the polyimide precursor solution, followed by drying ofthe coating film to form a coat containing the polyimide precursor andthe uncrosslinked resin particle, and a second step of heating the coatto imidize the polyimide precursor and form a polyimide film, the secondstep including a treatment for removing the uncrosslinked resin particlewith an organic solvent capable of dissolving the uncrosslinked resinparticle.
 5. A method for producing a resin particle-dispersed polyimideprecursor solution, comprising polymerizing a tetracarboxylic aciddianhydride and a diamine compound in a resin particle dispersion liquidwherein resin particles are dispersed in an aqueous solvent, in thepresence of an organic amine compound to form a polyimide precursor. 6.The method for producing a resin particle-dispersed polyimide precursorsolution as claimed in claim 5, wherein the resin particle dispersionliquid is a dispersion liquid wherein the resin particle is granulatedin the aqueous solvent.
 7. The method for producing a resinparticle-dispersed polyimide precursor solution as claimed in claim 5,wherein the organic amine compound is a tertiary amine compound.
 8. Themethod for producing a resin particle-dispersed polyimide precursorsolution as claimed in claim 5, wherein the organic amine compound is anamine compound having a nitrogen-containing heterocyclic structure.
 9. Aresin particle-dispersed polyimide precursor solution obtained by theproduction method claimed in claim 5.